Semiconductor laser control system

ABSTRACT

A pulse width modulation and intensity modulation signal generating unit, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal. An error amplifier forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal. A current driving unit causes a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current thereof, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop. The pulse width modulation and intensity modulation signal generating unit, the error amplifier and the current driving unit are formed as one chip of an integrated circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser control systemfor driving and controlling a semiconductor laser which is used as alight source in each of a laser printer, a digital copier, an opticaldisc drive device, an optical communications apparatus and so forth.

2. Description of Related Art

A semiconductor laser is very small, and it is possible to modulate alight output at high speed through a driving current. Therefore, asemiconductor laser is widely used recently as a light source of a laserprinter or the like.

However, because the relationship between a driving current and a lightoutput of a semiconductor laser varies greatly due to temperaturechange, it may be difficult to precisely set the light output of asemiconductor laser to a desired value. In order to solve this problemand to effectively use the advantages of a semiconductor laser, variousAPC (Automatic Power Control) electric circuits have been proposed.

The APC electric circuits can be broadly divided into the followingthree systems:

In a first system, the light output of a semiconductor laser ismonitored through a light reception device. A negative feedback loopalways controls the forward current of the semiconductor laser so that asignal in proportion to a light reception current which occurs in thelight reception device and is in proportion to the light output of thesemiconductor laser may be equal to a light emission instruction signal.Thus, the light output of the semiconductor laser may be controlled tobe a desired value. (Throuout the present application, the `negativefeedback loop` includes electricity-to-light conversion at a portionwhere a semiconductor laser emits light and light-to-electricityconversion at a portion where a light reception device receives thelight emitted by the semiconductor laser.)

In a second system, during a power setting period, the light output of asemiconductor laser is monitored through a light reception device; and anegative feedback loop always controls the forward current of thesemiconductor laser so that a signal in proportion to a light receptioncurrent which occurs in the light reception device and is in proportionto the light output of the semiconductor laser may be equal to a lightemission instruction signal. During the time other than the powersetting period, the forward current which has been set during the powersetting period is maintained, and thus, the light output of thesemiconductor laser is controlled to a desired value. Further, during atime other than the power setting period, the forward current which hasbeen set during the power setting period is modulated based oninformation, and thereby, the information is carried on the light outputof the semiconductor laser.

In a third system, the temperature of a semiconductor laser is measured,and thus, a temperature signal is obtained. Through the temperaturesignal, the forward current of the semiconductor laser is controlled, orthe temperature of the semiconductor is controlled to be fixed. Thereby,the light output of the semiconductor laser is controlled to a desiredvalue.

In order to obtain a desired value of light output, the first system ispreferable. However, in the first system, a control speed has a limitdue to the limits of the operation speed of the light reception device,the operation speed of an amplifying device which is used in thenegative feedback loop and so forth. For example, when the cutofffrequency f₀ of the negative feedback loop in the open loop condition isconsidered, the step response characteristics of the semiconductor lasercan be approximated, as follows:

    P.sub.out =P.sub.0 {1-exp(-2πf.sub.0 t)},

where:

P_(out) represents light output of the semiconductor laser;

P₀ represents a set light intensity of the semiconductor laser; and

t: represents time.

In many cases, it is required that the total light quantity (theintegral value of the light output: ∫P_(out) ·dt) until a set time τ₀has passed immediately after a light intensity of the semiconductorlaser was changed should be a predetermined value, where:

    ∫P.sub.out ·dt=P.sub.0 ·τ.sub.0 {1- 1/(2πf.sub.0 τ.sub.0)! 1-exp(-2πf.sub.0 τ.sub.0)!}.

Assuming that τ₀ =50 (ns) and an error allowance is 0.4%, it should bethat f₀ >800 (MHz), and it is very difficult to satisfy this condition.

In the second system, the above-described problem occurring in the firstsystem does not occur, and thus, it is possible to modulate the lightoutput of the semiconductor laser at high speed. Therefore, the secondsystem is widely used. However, according to the second system, thelight output of the semiconductor laser is not always controlled.Therefore, an external disturbance or the like may easily cause thelight intensity of the semiconductor laser to vary. For example, the Doloop characteristics of a semiconductor laser may easily cause the lightintensity of the semiconductor laser to include an error of severalpercent. As an attempt to restrict the Do loop characteristics of asemiconductor laser, a method has been proposed in which the heat timeconstant of the semiconductor laser is matched by the frequencycharacteristics of a semiconductor laser driving current and thus the Doloop characteristics are compensated. However, the heat time constant ofa semiconductor laser varies among respective particular semiconductorlasers, and also, it varies due to an ambient condition. Therefore, sucha method may not be effective.

For example, the applicant of the present invention proposed animprovement in consideration of such a problem in Japanese Laid-OpenPatent Application No.2-205086 (corresponding U.S. Pat. No. 5,036,519).According to the proposed method, as shown in FIG. 1, a light receptiondevice 2 monitors the light output of a semiconductor laser 1. Anegative feedback loop 3 always controls the forward current of thesemiconductor laser 1 so that an output signal of the light receptiondevice 2 may be equal to a light emission instruction signal (DATA). Acurrent driving unit 4 converts the light emission instruction signal(DATA) into the forward current of the semiconductor laser 1. The lightoutput of the semiconductor laser 1 is controlled through the currentwhich is the sum (or difference) of a control current of the negativefeedback loop 3 and a driving current generated by the current drivingunit 4. In the example shown in FIG. 1, the negative feedback loop 3includes the semiconductor laser 1, the light reception device 2, aconstant-current source 5 of a constant current I_(DA1) and an invertingamplifier 6. The output of the inverting amplifier 6 is used to driveand control a driving transistor 7. The semiconductor laser 1, thedriving transistor 7 and a resistor Re are connected in series as shownin the figure. The current driving unit 4 includes a constant-currentsource 8 of a constant current I_(DA2).

In the circuit configuration, when light output corresponding to acurrent through which the current driving unit 4 directly drives thesemiconductor laser 1 is referred to as P_(S), the step responsecharacteristics of the light output of the semiconductor laser can beapproximated as follows:

    P.sub.out =P.sub.0 +(P.sub.S -P.sub.0){1-exp(-2πf.sub.0 t)}.

When P_(S) ≈P₀), the light output of the semiconductor laser 1immediately becomes equal to P₀. Therefore, f₀ may have a relativelysmall value in comparison to the case where there is only the negativefeedback loop 3. FIG. 2A shows how the light output changes only throughthe negative feedback loop 3 (control unit). FIG. 2B shows how the lightoutput changes in the case where the constant current I_(DA2) is addedby the current driving unit 4. In a practical case, f₀ may have a valueof approximately 40 (MHz). Such an amount of cutoff frequency f₀ can beeasily obtained.

The applicant of the present invention also disclosed a semiconductorlaser control system in Japanese Laid-Open Patent Application No.5-67833(corresponding U.S. Pat. No. 5,237,579). In the disclosed system,bipolar transistors are used as elements of the configuration ofJapanese Laid-Open Patent Application No.2-205086 described above, andthus an IC is formed. Thereby, it is easy to design a negative feedbackloop.

A one-dot multi-level method will now be described, in a case where alaser printer is taken as an example. A laser printer was developed as anon-impact printer for taking the place of a line printer. Because of ahigh speed printing characteristic and a high resolution characteristicof the laser printer, application of the laser printer to an imageprinter was attempted. As a result, various printing methods, which usea dither method as basic technology, have comes to be practically usedin laser printers. Further, as a result of recent quick development ofsemiconductor technology, the amount of information which can beprocessed by a laser printer has quickly increased. AS a result, in alaser printer, a one-dot multi-level method is practically used, andthereby, a laser printer is effectively used as an image printer. In theone-dot multi-level method, in a high-end machine, the number of tonelevels is that which can be obtained in the use of 8 bits. However, in alow-end machine, the number of tone levels is a low number (severallevels). One reason is that the information amount to be processedincreases when the number of tone levels increases. However, a mainreason is that the scale and costs of the electrical circuit of asemiconductor laser control modulation unit increases when the number oftone levels increases.

Currently, the following three semiconductor laser control modulationmethods have been proposed:

a light intensity modulation (power modulation) method;

a pulse width modulation method; and

a pulse width and intensity combined modulation method.

The light intensity modulation (power modulation, which may beabbreviated to `PM`) method will now be described. In the method, lightoutput itself is changed when a dot is printed. In this method, a middleexposure range is used for obtaining middle tone levels. Therefore, itis important that a printing process should be stabilized, and thusrequirements for the printing process are strict. However, in thismethod, semiconductor laser control modulation can be easily performed.

The pulse width modulation (which may be abbreviated to `PWM`) methodwill now be described. In this method, there are two light outputlevels. A time of light emission is changed (that is, a pulse width of alight emission instruction signal is changed) when a dot is printed.Therefore, in comparison to the PM method, the middle exposure range isless used. Further, by coupling adjacent dots, it is possible to furtherdecrease use of the middle exposure range. As a result, requirements forprinting process stability are reduced. However, when pulse widthsetting is performed with 8-bit data and coupling of adjacent dots isperformed, the configuration of a semiconductor laser control modulationunit is complex.

The pulse width and intensity combined modulation method (PWM+PM method)will now be described. As mentioned above, requirements for a printingprocess are strict in the PM method and a semiconductor laser controlmodulation unit is complex in the PWM method. In order to solve theproblems, the pulse width and intensity combined method was considered.For example, the applicant of the present invention discloses thismethod in Japanese Laid-Open Patent Application No.6-347852(corresponding U.S. patent application Ser. No. 08/253,322), nowabandoned and refiled as U.S. Ser. No. 08/862,326.

This modulation method is basically a two-level printing method, andthus is a method using the PWM method, in which requirements forprinting process stability are not strict, as a basic technology. The PMmethod is used for interpolate a change of a pulse width. When the sameresolution is obtained, in comparison to a case of each separatemodulation method (the PM and PWM methods), each of the number of pulsewidths and the number of power levels can be reduced. This is because,in the combined modulation method, the resolution is provided as aresult of combining the number of pulse widths and the number of powerlevels. As a result, it is possible to easily provide an arrangementrequired for each separate modulation method. Thereby, requirements forprinting process stability are not strict, and also it is suitable toprovide an integrated circuit which performs the combined modulationmethod. Thus, it is possible to miniaturize and reduce costs of anarrangement required for performing the combined modulation method.

FIG. 3 shows an example of an arrangement of a semiconductor lasercontrol system which performs the combined modulation method. In thearrangement, image data and an input clock signal are input to a pulsewidth generating unit and data modulation unit 11. The pulse widthgenerating unit and data modulation unit 11 outputs a light emissioninstruction signal (DATA) to a semiconductor laser control unit andsemiconductor laser driving unit 12. The semiconductor laser controlunit and semiconductor laser driving unit 12 has, for example, a circuitconfiguration such as that shown in FIG. 1. According to the input imagedata, the pulse width generating unit and data modulation unit 11performs basically the PWM method, and the PM method for interpolating achange of a pulse width.

A basic concept of light output waveforms of the semiconductor laser 1is shown in FIG. 4. For the sake of simplicity of description, in theexample shown in FIG. 4, there are three pulse widths and six powerlevels. Thereby, a total of 18 tone levels are output. FIG. 4 typicallyshows light output waveforms in this case. As shown in the figure, inthis combined modulation method, basically the PWM method is used. Powermodulation using a middle exposure range is performed on a minimum pulsewidth. In order to obtain those light output waveforms, for example, asshown in FIGS. 5A and 5B, either a pulse 1 having a pulse width of T anda pulse 2 having a pulse width of T+δT (δT being the minimum pulsewidth) or a pulse 3 having a pulse width of T and a pulse 4 having apulse width of δT are generated. For the pulse of the pulse width of T,each of the bits is of the H level, while, for the pulse of the pulsewidth δT (in the case of the pulse 2, during the time δT only the pulse2 is at the high level), respective bits may be of either the H level orthe L level according to the PM data. Thereby, the light outputwaveforms shown in FIG. 4 and FIGS. 5A, 5B can be provided. In theexample shown in FIG. 5A, a pulse width starts from the left side, and,in the example shown in FIG. 5B, a pulse width starts from the rightside.

For example, Japanese Laid-Open Patent Application No.6-347852 disclosesthat, for implementing such a pulse width and intensity combinedmodulation system within one dot, a pulse width generating unit isformed as an IC using C-MOS devices and thus the unit is easilyprovided, and also, a negative feedback loop unit is formed as an ICusing bipolar transistors and is easily designed.

However, methods disclosed in Japanese Laid-Open Patent ApplicationNo.6-347852 may be further improved. It is considered that a currentadding method by which a control amount of the negative feedback loopcan be effectively reduced and a pulse width and intensity combinedmodulation method in a pulse (pulses) within one dot may be implementedby a further miniaturized and increased power saving configuration, andalso may function at higher speed and with higher accuracy.

When harnesses or the like are used for data transfer, there areproblems which will be described below. For example, in a case,disclosed in Japanese Laid-Open Patent Application No.6-347852, where apulse width generating unit and a data modulation unit, to which inputdata is input, are formed as C-MOS devices, because the operationalprinciple of C-MOS devices is generally a switching operation (that is,an turning on-turning off operation), it is necessary that the inputlevel of the input data should be 0 to 5 volts or 0 to 3 volts. In atransmission path such as a harness or the like, characteristicimpedance Z of the line is expressed as:

    Z=(L/C).sup.1/2.

Assuming that L=10 (mH/cm) and C=0.7 (pF/cm), Z≈120 (Ω). FIG. 6 shows anexample of input data using a harness 15 in a case of an integratedcircuit 14 using a C-MOS device. For example, input data is input to theintegrated circuit 14 via a connecting point at which voltage dividingresistors R_(A) (330 Ω) and R_(B) (220 Ω) are connected with oneanother. In the integrated circuit 14, a constant-current source 16 isincluded as shown in the figure. Assuming that the length of the harness15 is 50 cm, a rising time constant τ of a data waveform of input dataduring data transfer through the harness 15 is expressed as:

    τ=CR=35(pF)·120(Ω)=4(ns).

Considering 10 to 90% of the full variation range of the data, as shownin FIG. 7,

    t≈2.5τ=10(ns).

Thereby, when data transfer is performed in a synchronization condition,it is possible to perform data transfer at 25 (MHz) (corresponding to 40ns) at the highest, and it is difficult to perform data transfer at ahigher speed than 25 (MHz). When the pulse width generating unit anddata modulation unit are formed simply with bipolar transistors,generally speaking, an input level of input data is set to 0 to 5 voltsor 0 to 3 volts in consideration of an interface in a case where such anelectronic circuit may be connected with a C-MOS device. Therefore, thesame problem may occur, and it is difficult to perform data transfer athigher speed. Further, because the amount of voltage swing of the inputis large as mentioned above, taking EMI (Electro-Magnetic Interference)measures and achieving power saving may be difficult.

Further, in such a semiconductor laser control system, a current in alight emission instruction signal generating unit is, when consideringonly a direct current component, a monitoring current of a monitoringlight reception device. Therefore, when the electronic circuit is formedas an integrated circuit, it is necessary that the current in the lightemission instruction signal should be a current which does not varydepending on changes of the integrated circuit internal temperature.When no special measures are taken therefor, stabilization of themonitoring current may cause problems. When stabilization of themonitoring current is not ensured, the semiconductor laser controlsystem cannot perform an adequate operation for a wide range of themonitoring current.

Further, when a control speed is uniformly set for a control system ofthe negative feedback loop, flexibility in designing the control systemmay be reduced and it is not possible to freely set the control speed toa desired speed.

For the purpose of obtaining, under high-speed control, an idealwaveform of light output which is always optimized, it is important toappropriately set the level P_(S) shown in FIG. 2B, and thereby to causethe light output waveform to approximate that of a rectangular wave.This is important, in particular, in a case where modulation isperformed for a larger number of tone levels using pulse width andintensity combined modulation method which was described with referenceto FIGS. 4, 5A and 5B.

As general characteristics of a semiconductor laser, there is a currentchange due to a temperature change shown in FIG. 8, and also, there is acurrent change due to an elapsing time change (especially, a change of adifferential quantum efficiency) shown in FIG. 9. For the characteristicof an operation current change due to temperature change, asemiconductor laser control system can appropriately respond thereto bycausing a negative feedback loop 3 such as that shown in FIG. 1 toalways operate, whereby, even when the oscillation threshold current Ithof the semiconductor laser varies, the control system follows it.Thereby, the control system causes the oscillation threshold current Ithto flow through the semiconductor laser 1 as its forward current.

However, as shown in FIG. 9, when there is a change of an operationcurrent due to elapsing time, especially due to a differential quantumefficiency, the change in an operation current is larger than that in anoperation current due to a temperature change. Such changecharacteristics due to a differential quantum efficiency affects theP_(S) level shown in FIG. 2B. Thereby, actual light output is too highin comparison to a desired light output P_(out), and thus an overshootoccurs, or actual light output is too low in comparison to a desiredlight output P_(out), and thus an undershoot occurs. Thus, high-speedcontrol cannot be achieved.

In the related art, a problem may be the lack of accuracy in detectionof a differential quantum efficiency of a semiconductor laser, andadaptability thereto may be insufficient. Thereby, it may be difficultto provide a light output waveform which approximates that of arectangular wave.

SUMMARY OF THE INVENTION

According to the present invention, a semiconductor laser control systemcomprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and thus generates a light emission instructionsignal;

an error amplifier which forms a negative feedback loop with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit which provides a driving current, according tothe light emission instruction signal, to flow through the semiconductorlaser as the forward current thereof, the driving current beinggenerated so as to control driving of the semiconductor laser with acurrent of the difference or sum with the control current of thenegative feedback loop,

wherein the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit areformed to be one chip of an integrated circuit.

All of the arrangement including a digital control system of the pulsewidth modulation and intensity modulation signal generating unit and ananalog driving system of the error amplifier and current driving unit isformed to be one chip of an integrated circuit. Thereby, miniaturizationand energy saving can be achieved while the combined pulse width andintensity modulation methods for one dot can be performed with highspeed and high accuracy.

It is preferable that the pulse width modulation and intensitymodulation signal generating unit comprises:

data converting means for converting the input data into pulsemodulation data and intensity modulation data;

pulse width modulation means which, based on the pulse modulation data,generates a plurality of pulse-modulated pulses; and

a light emission instruction signal generating unit which, based on theoutputs of the data converting means and the pulse width modulationmeans, performs the pulse width modulation and intensity modulation andgenerates the light emission instruction signal for the semiconductorlaser.

A detailed arrangement of the pulse width modulation and intensitymodulation signal generating unit which forms the digital control systemis clarified.

The one chip of the integrated circuit may be formed by bipolartransistors. Thereby, especially, an amplifier of the analog drivingsystem such as the error amplifier and current driving unit can beeasily formed. Further, an input level therefor can be freely set andcan be advantageously reduced.

The one chip of the integrated circuit may be formed by C-MOStransistors. Thereby, the pulse width modulation and intensitymodulation signal generating unit can be easily formed and it ispossible to further miniaturize the integrated circuit.

The one chip of the integrated circuit may provided by a compositecircuit of bipolar transistors and C-MOS transistors. Thereby, anamplifier of the analog driving system such as the error amplifier andcurrent driving unit can be easily formed by bipolar transistor, whilethe digital control system such as the pulse width modulation andintensity modulation signal generating unit can be easily formed byC-MOS transistors. Thus, circuit design can be easily performed.

One object of the present invention is to achieve high-speedhigh-reliability data transfer by effectively using features of bipolartransistor circuits in forming a semiconductor laser control system tobe an integrated circuit.

For this purpose, according to the present invention, a semiconductorlaser control system, comprises:

pulse width modulation and intensity modulation generating unit which,based on input data, performs pulse width modulation and intensitymodulation and generates a light emission instruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

wherein:

the pulse width modulation and intensity modulation signal generatingunit, the error amplifier and the current driving unit are provided bybipolar transistor as one chip of an integrated circuit; and

an emitter coupled logic circuit included in an input portion of thepulse width modulation and intensity modulation signal generating unitto receive the input data.

When an integrated circuit is formed by bipolar transistors, byproviding the emitter coupled logic in the portion in which input datais received, a logic operation can be established, for example, with avoltage swing of approximately 500 mV. Thus, input voltage swing can bereduced, thereby a waveform rising time constant can be reduced, andhigh-speed transfer can be achieved. Further, input energy can begreatly reduced and measures against EMI, energy saving can be achieved.

Further, not only the pulse width modulation and intensity modulationsignal generating unit, the error amplifier and the current driving unitare formed to be one chip of an integrated circuit by bipolartransistors but also an input portion of the pulse width modulation andintensity modulation signal generating unit to which the input data istransmitted is formed with an impedance matching circuit. Thereby,reflection at a data transfer portion can be avoided, and high-speeddata transfer, and measures against EMI and energy saving can beachieved.

Not only the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit areformed to be one chip of an integrated circuit by bipolar transistors,but also the input data is input to the pulse width modulation andintensity modulation signal generating unit in a combination of apositive logic signal and a negative logic signal, through two lines inparallel. Thereby, the voltage swing at the input portion can be furtherreduced and measures against EMI and energy saving can be achieved. Evenif noise is included in an input signal, the noise is canceled becauseboth the positive logic signal and negative logic signal are similarlyaffected by the noise. Thus, noise-resistant data transfer can beachieved.

By appropriately combining the above-mentioned arrangement, the totalityof the advantages thereof can be obtained.

Not only the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit areformed by bipolar transistors to be one chip of an integrated circuit,but also the pulse width modulation and intensity modulation signalgenerating unit comprises a light emission instruction signal generatingunit which performs pulse width modulation and intensity modulation andgenerates the light emission instruction signal for the semiconductorlaser, an external connection device being provided for setting thecurrent value of the light emission instruction signal generating unit.

The current of the light emission instruction signal generating unit isequal to the monitor electric current of the light reception device in asteady state. Therefore, it is necessary to prevent the current frombeing affected by the temperature in the integrated circuit. However, byadjusting the external connection device for adapting the control systemto the characteristics of the semiconductor laser and light receptiondevice, the monitor current can be stabilized so that a desired lightoutput is obtained. When the monitor current varies, it is possible toadapt the control system to the monitor current.

Not only the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit areformed by bipolar transistors to be one chip of an integrated circuit,but also an external connection device is provided for setting a controlspeed of the negative feedback loop.

Flexibility in the control system design is improved, and the controlspeed can be freely set to a desired value.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit for causing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of the semiconductor laser;

a memory unit for storing a detection result of the differential quantumefficiency detecting unit;

an adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection result storedin the memory unit; and

a timing generating unit,

wherein, in initialization, the timing generating units generates atiming signal sufficiently slower than the control speed of the erroramplifier, the differential quantum efficiency detecting unit detectsthe differential quantum efficiency of the semiconductor laser based onthe timing signal, the memory units stores a detection result at eachtiming, and the current corresponding to the light emission instructionsignal is set using the stored detection results.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of the semiconductor laser;

a timing generating unit for generating a timing signal which controls adetection operation of the differential quantum efficiency detectingunit in initialization;

a memory unit for storing a detection result of the differential quantumefficiency detecting unit at each timing; and

an adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection resultsstored by the memory unit.

If there is a change in the differential quantum efficiency of thesemiconductor laser due to elapsing time of use, the change is detectedand a current adding value can be again set to an optimum value whenpower supply is started or a reset operation is performed. Thereby, theamount of control performed by the control unit, that is, the negativefeedback loop can be greatly reduced. Accordingly, the semiconductorlaser light output waveform can be close to an ideal rectangular wave orsubstantially a rectangular wave without an overshoot or slow rising,and optimum light output can be always obtained.

It is possible that the pulse width modulation and intensity modulationsignal generating unit, the error amplifier, the current driving unit,the differential quantum efficiency detecting unit, the memory unit, theadding current setting unit and the timing generating unit are formed tobe one chip of an integrated circuit. Also, it is possible that the chipis formed by bipolar transistors. Thereby, energy saving andminiaturization of the system can be achieved.

It is preferable that:

the current driving unit comprises a voltage shifting unit in the erroramplifier, includes a differential circuit for changing the amount ofvoltage shift, and is provided in the negative feedback loop;

the adding current setting unit sets a current of the differentialcircuit so that light output of the semiconductor laser is a desiredmaximum value when a current corresponding to the light emissioninstruction signal is maximum, and light output of the semiconductorlaser is a desired minimum value when a current corresponding to thelight emission instruction signal is minimum;

in initialization, light output of the semiconductor laser is set to thedesired maximum value at a certain timing T0, light output of thesemiconductor laser is set to the desired minimum value at a timing T1after a fixed time has elapsed from the timing T0, the differentialquantum efficiency detecting unit and the adding current setting unitare operated and the current is set between the timing T1 and a timingT2 after a fixed time has elapsed from the timing T1.

Thus, a desired maximum amount of the high-speed voltage shifting isdetected, and a light output is achieved to be close to an idealrectangular wave or a substantially a rectangular wave.

The timing generating unit may include an external connection device forgenerating timing signals. The timing generating unit includes, forexample, delay circuits. Even in such a case, a timing signals can begenerated using the external connection device. Therefore, the controlspeed of the negative feedback loop can be freely set. The timingsignals may be generated using the external connection circuit takingthe thus-set control speed of the negative feedback loop intoconsideration. The timing signals may be generated also taking thefrequency characteristics of the semiconductor laser and light receptiondevice into consideration so that light output without includinginfluence of the frequency characteristics of the semiconductor laserand light reception device can be obtained in the initializationoperation using the timing signals T₀, T₁, T₂, T₃, T₄, and T₅. By thusappropriately generating the timing signals, the time required for theinitialization operation using the timing signals T₀, T₁, T₂, T₃, T₄,and T₅ can be set as being an optimum time.

The timing generating unit may include an oscillation circuit and maygenerate a plurality of timing signals based on the oscillation outputof the oscillation circuit. By using an oscillation circuit in thetiming generating unit, even if it is necessary to generate manytimings, the timings can be freely set using only an external connectioncapacitor. Further, by appropriately setting the capacitance of thecapacitor, it is not necessary to provide a circuit for compensating anadverse effect due to the frequency characteristics of the semiconductorand light reception device.

The timing generating unit may include an oscillation circuit and manystages of latch circuits, and the respective latch circuits maygenerates timing signals based on the oscillation output of theoscillation circuit. Thus, a combination of latch circuits, notflip-flops, are used for the oscillation circuit. Thereby, it ispossible to effectively reduce the number of components.

In the related art, when performing the differential quantum efficiencydetection, appropriate image data (that is, a specific light emissionlevel instruction signal) should be input. Thus, setting of the lightoutput Ps as a result of the differential quantum efficiency detectionby only internal processing of the system without regard to input imagedata cannot be performed in the related art.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of the semiconductor laser;

a memory unit for storing a detection result of the differential quantumefficiency detecting unit;

an adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection result storedin the memory unit;

a timing generating unit; and

a switch unit, to which a forcible light emission instruction signal anda forcible light cessation instruction signal are selectively input, theswitch unit providing an output selected from outputs including thelight emission instruction signal based on input data;

wherein:

the pulse width modulation and intensity modulation signal generatingunit, the error amplifier, the current driving unit, the differentialquantum efficiency detecting unit, the memory unit, the adding currentsetting unit and the timing generating unit are formed with bipolartransistors as one chip of an integrated circuit; and

in initialization, the timing generating units generates a timing signalsufficiently slower than the control speed of the error amplifier, thedifferential quantum efficiency detecting unit detects the differentialquantum efficiency of the semiconductor laser based on the timingsignal, the memory units stores a detection result at each timing, andthe current corresponding to the light emission instruction signal orthe forcible light emission instruction signal is set using the storeddetection results.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit which causes a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

a switch unit, to which a forcible light emission instruction signal anda forcible light cessation instruction signal are selectively input, theswitch unit providing an output selected from outputs including thelight emission instruction signal based on input data;

a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of the semiconductor laser based on atiming signal;

a timing generating unit for generating a timing signal which issufficiently slower than the control speed of the error amplifier, forcontrolling a detection operation of the differential quantum efficiencydetecting unit, in initialization;

a memory unit for storing a detection result of the differential quantumefficiency detecting unit at each timing; and

an adding current setting unit for setting a current, corresponding tothe light emission instruction signal or the forcible light emissioninstruction signal, using the detection results stored by the memoryunit.

If there is a change in the differential quantum efficiency of thesemiconductor laser due to an elapsing time of use, the change isdetected and a current adding value can be again set to an optimum valuewhen power supply is started or a reset operation is performed. Anamount of control performed by the control unit, that is, the negativefeedback loop can be greatly reduced. Accordingly, the semiconductorlaser light output waveform can be close to an ideal rectangular wave ora substantially rectangular wave without an overshoot or slow rising,and optimum light output can be always obtained. Further, the switchunit is provided, to which one of the forcible light emissioninstruction signal and forcible light cessation instruction signal isselectively input, and the switch unit provides an output selected fromoutputs including the light emission instruction signal based on inputdata. When detecting the differential quantum efficiency, thesemiconductor laser can be forcibly turned on and turned off. Thus, thedifferential quantum efficiency detection can be performed withoutneeding input data. By using the switch unit, the differential quantumefficiency detection can be performed with only internal processing.Then, by using a result of the differential quantum efficiencydetection, light output can be optimized.

It may be possible that the pulse width modulation and intensitymodulation signal generating unit comprises:

a pulse generating means for generating a plurality of pulses having afrequency the same as the frequency of an input clock signal and havingdifferent phases, the phase difference being a fixed phase difference;

data converting means for converting input data into pulse widthmodulation data and power modulation data; and

pulse width modulation means for generating a plurality of pulses whichhave undergone pulse width modulation, based on the pulse widthmodulation data, from the pulses generated by the pulse generatingmeans.

In a relatively simple arrangement, the pulse width modulation andintensity modulation signal generating unit can be formed.

The pulse width modulation and intensity modulation signal generatingunit, the error amplifier, the current driving unit, the switch unit,the differential quantum efficiency detecting unit, the timinggenerating unit, the memory unit and the adding current setting unit maybe one chip of an integrated circuit.

In the related art, the current driving unit and error amplifier whichis a part of the negative feedback loop (corresponding to the currentdriving unit 4 and inverting amplifier 6 shown in FIG. 1, for example),each controlling a current flowing to the semiconductor laser, areprovided as separate portions. As a result, the number of requiredcomponents is large. Accordingly, power consumption is also large.Therefore, it is difficult to further miniaturize the integratedcircuit.

One object of the present invention is to improve an arrangement of theerror amplifier and current driving unit so that further miniaturizationmay be accomplished, and to provide a peripheral arrangement for makingthe best use of the thus-improved arrangement of the error amplifier andcurrent driving unit.

According to another aspect of the present invention, a semiconductorlaser control system comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit which causes a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

wherein:

the pulse width modulation and intensity modulation signal generatingunit, the error amplifier and the current driving unit are formed as onechip of an integrated circuit: and

the current driving unit is provided in the negative feedback loop.

The current driving unit is integrated with the error amplifier andincorporated into the negative feedback loop. This portion can beminiaturized, and the integrated circuit can be further miniaturized.Especially, it is possible that a large current driving part is only onepart. Thereby, energy saving can be performed and large-scaleintegration can be achieved. Because it is only needed to drive a smallcurrent, high-speed driving can be achieved.

In the related art, when a bipolar transistor arrangement suitable foran amplifier arrangement is used as a base of the system, it is requiredto make the best use of the features of the bipolar transistorarrangement. For this purpose, the above-described arrangement accordingto the present invention may be formed by bipolar transistors.

An external connection device may be provided for setting a currentvalue of the pulse width modulation and intensity modulation signalgenerating unit. In a steady state, the current of the pulse widthmodulation and intensity modulation signal generating unit is equal tothe monitor current of the light reception device. Therefore, it isnecessary that the current of the pulse width modulation and intensitymodulation signal generating unit is a current which is not affected bytemperature variation in the integrated circuit. However, by adjustingthe external connection device, it is possible to set the current of thepulse width modulation and intensity modulation signal generating unit,taking the characteristics of the semiconductor laser and lightreception device into consideration. Thereby, a desired light output canbe obtained. By appropriately changing the value of the externalconnection device, the system can be adapted to a wide range of themonitor current of the light reception device.

An external connection device may be provided for setting the controlspeed of the negative feedback loop. Thereby, flexibility in designingthe control system can be improved, and it is possible to freely set thecontrol speed negative feedback loop.

It is preferable that the current driving unit is a high-speed shiftingunit in the error amplifier, and a differential circuit for changing theamount of voltage shifting of the high-speed shifting unit is includedin the current driving unit. The current driving unit is provided in thenegative feedback loop. Voltage driving using the high-speed voltageshifting unit enables further high-speed control of light output.Further, setting of the voltage is easily performed.

It is preferable that there are common components among a plurality ofdigital to analog converters in a light emission instruction signalgenerating unit of the pulse width modulation and intensity modulationsignal generating unit. Thereby, the number of components can beeffectively reduced, and further energy saving can be achieved.

An offset setting unit may be provided which includes an externalconnection device for setting an offset current which is used forcausing light output of the semiconductor laser to be of a desiredminimum value. In order to perform light output control in the negativefeedback loop in real time, light output of the semiconductor lasershould not be completely zero. Therefore, it is necessary to set theoffset current for causing light output of the semiconductor laser to beof the desired minimum value. By using the external device, the desiredoffset current can be easily set.

A combined setting unit may be provided having an external connectiondevice for combined setting of a maximum current of the pulse widthmodulation and intensity modulation signal generating unit and an offsetcurrent which causes light output of the semiconductor laser to be aminimum value. The offset current can be appropriately automatically setby only setting the maximum current of the pulse width modulation andintensity modulation signal generating unit using the externalconnection device.

An offset setting unit may be provided which has an external connectiondevice for setting an offset current which is used for causing lightoutput of the semiconductor laser to be a desired minimum value; and

a combined setting unit may be provided having an external connectiondevice for combined setting of a maximum current of the pulse widthmodulation and intensity modulation signal generating unit and theoffset current.

The offset current can be automatically set appropriately by onlysetting the maximum current of the pulse width modulation and intensitymodulation signal generating unit using the external connection device.Also, using the offset setting unit, the offset current can be freelyadjusted.

A starting-up unit may be provided for allowing start a operation when apower source voltage reaches a predetermined voltage in power supplystarting. If the initialization operation, current driving unit settingoperation and so forth are performed before a power source voltagereaches a predetermined voltage in a power supply starting condition, adesired light output of the semiconductor laser may not be provided. Thestarting-up unit allows starting of these operations after the powersupply voltage has reached the predetermined voltage. The initializationof the integrated circuit can be performed with high accuracy, and also,the semiconductor laser can be protected when the system is starting.

A light emission instruction signal generating unit which causes anabsolute current determined by the light reception device to flow, mayhave a base current compensation unit for compensating base currents oftransistors connected to a path of a reference current. Thereby, errorcurrents due to base currents of transistors can be prevented fromoccurring, and transistor characteristic changes due to such errorcurrents can be prevented. Thus, the current of the light emissioninstruction signal generating unit can be provided with high accuracy.

An external connection device may be provided for setting a currentvalue of the pulse width modulation and intensity modulation signalgenerating unit; and

a light emission instruction signal generating unit which causes anabsolute current determined by the light reception device to flow, mayhave a base current compensation unit for compensating base currents oftransistors connected to a path of a reference current.

The current of the pulse width modulation and intensity modulationsignal generating unit can be adjusted taking the characteristics of thesemiconductor laser and light reception device into consideration. Thus,a desired light output can be obtained. Furthermore, error currents dueto base currents of transistors can be prevented from occurring, andtransistor characteristic changes due to such error currents can beprevented. Thus, the current of the light emission instruction signalgenerating unit can be provided with high accuracy.

A plurality of the pulse width modulation and intensity modulationsignal generating units may be provided. The system can be adapted to awide range of the monitor current, and can be adapted to various typesof semiconductor lasers and light reception devices having differentspecifications.

An input terminal of an external control voltage for changing a maximumcurrent of the pulse width modulation and intensity modulation signalgenerating unit may be provided. The maximum current of the pulse widthmodulation and intensity modulation signal generating unit can bedynamically changed using an external control voltage. Thus, a shadingcorrection operation during a normal operation of the system and slightadjustment of light intensity can be performed.

A starting-up unit may be provided for allowing a start of operationwhen a light emission instruction current corresponding to the lightemission instruction signal reaches a predetermined current in powersupply starting. Thereby, similar to case of the above-mentionedstarting-up unit for the power source voltage, the initializationoperation can be performed with high accuracy, and also, thesemiconductor laser can be protected when the system is starting.

An adding current setting unit for setting a driving currentcorresponding to the light emission instruction signal and a combinedchanging unit for causing a maximum current of the pulse widthmodulation and intensity modulation signal generating unit and a maximumcurrent of the adding current setting unit to change together may beprovided. When performing the above-mentioned dynamic change of themaximum current of the pulse width modulation and intensity modulationsignal generating unit for performing the shading correction operation,the maximum current of the addition current setting unit isautomatically changed at the same time. It is possible that a lightoutput waveform is close to an ideal rectangular wave, or issubstantially a rectangular wave.

A starting-up unit for allowing an operation start when a light emissioninstruction current corresponding to the light emission instructionsignal reaches a predetermined current in power supply starting, anadding current setting unit for setting a driving current correspondingto the light emission instruction signal and a combined changing unitfor causing a maximum current of the pulse width modulation andintensity modulation signal generating unit and a maximum current of theadding current setting unit to change at the same time may be provided.When performing the above-mentioned dynamic change of the maximumcurrent of the pulse width modulation and intensity modulation signalgenerating unit for performing the shading correction operation, themaximum current of the addition current setting unit is automaticallychanged at the same time. Thereby, it is possible that a light outputwaveform is close to an ideal rectangular wave, or is substantially arectangular wave.

A first starting-up unit for allowing an operation start when a powersource voltage reaches a predetermined voltage in power supply startingand a second starting-up unit for allowing an operation start when alight emission instruction current corresponding to the light emissioninstruction signal reaches a predetermined current in a starting powersupply may be provided. Similar to the case of using only theabove-mentioned starting-up unit for the power source voltage, and tothe case of using only the above-mentioned starting-up unit for thelight emission instruction current, the initialization operation can beperformed with high accuracy, and also, the semiconductor laser can beprotected when the system is starting.

In the related art, when the system is applied to a printer or a copier,a higher performance is demanded, for example, for achieving one-dotmulti-level output.

For example, the monitor current of the light reception device formonitoring light output of the semiconductor laser varies depending on aparticular product. Further, it is required to take the temperaturecharacteristics of the device into consideration and flexibility indesigning the control system, that is, the negative feedback loop shouldbe improved. It is required that the control speed of the negativefeedback loop is freely set. Accuracy in an initialization operation ofan considerably overall miniaturized integrated circuit should beconsidered. Protection of the semiconductor laser should also beconsidered.

One object of the present invention is to provide one chip of anintegrated circuit of a semiconductor laser control system whichfulfills the above-described demands which arise when being applied to aprinter, a copier or the like.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich comprises data converting means for converting the input data intopulse modulation data and intensity modulation data, pulse widthmodulation means which, based on the pulse modulation data, generates aplurality of pulse-modulated pulses, and a light emission instructionsignal generating unit which, based on the outputs of the dataconverting means and the pulse width modulation means, performs thepulse width modulation and intensity modulation and generates a lightemission instruction signal for the semiconductor laser, the pulse widthmodulation and intensity modulation signal generating unit, based oninput data, performing pulse width modulation and intensity modulationand generates the light emission instruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

wherein:

the pulse width modulation and intensity modulation signal generatingunit, the error amplifier and the current driving unit are formed as onechip of an integrated circuit: and

an external connection device is provided for setting a current value ofthe light emission instruction signal generating unit.

In a steady state operation, the current of the light emissioninstruction signal generating unit is equal to the monitor current ofthe light reception device. Therefore, it is necessary that the currentof the light emission instruction signal generating unit is a currentwhich is not affected by temperature variation in the integratedcircuit. However, by adjusting the external connection device, it ispossible to set the current of the light emission instruction signalgenerating unit by taking the characteristics of the semiconductor laserand light reception device into consideration. A desired light outputcan be obtained. By appropriately changing the value of the externalconnection device, the system can be adapted to a wide range of themonitor current of the light reception device. Further, the system canbe formed as an integrated circuit without any problems.

Not only the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit isformed as one chip of an integrated circuit, but also

an external connection device may be provided for setting the controlspeed of the negative feedback loop.

Flexibility in designing the control system is improved, the controlspeed can be freely set, and the system can be formed as an integratedcircuit without any problems.

A starting-up unit may be provided for allowing a start of operationwhen a power source voltage reaches a predetermined voltage in powersupply starting, and the pulse width modulation and intensity modulationsignal generating unit, the error amplifier, the current driving unitand the starting-up unit may be formed as one chip of an integratedcircuit.

As described above, if the initialization operation, current drivingunit setting operation and so forth are performed before a power sourcevoltage reaches a predetermined voltage in a power supply startingcondition, a desired light output of the semiconductor laser may not beprovided. The starting-up unit allows starting of these operations afterthe power supply voltage has reached the predetermined voltage. Thereby,the initialization of the integrated circuit can be performed with highaccuracy, and also, the semiconductor laser can be protected when thesystem is starting.

The starting-up unit may have a reset function for initializing theintegrated circuit by a control signal external of the integratedcircuit. By using the reset function, it is possible to again performingthe initialization operation on the integrated circuit. Thus, it ispossible to always provide a satisfactory light output.

The pulse width modulation and intensity modulation signal generatingunit may include a plurality of light emission instruction signalgenerating units. The system can be adapted to a wide range of monitorcurrent, and can be adapted to various types of semiconductor lasers andlight reception devices having different specifications.

An input terminal may be provided for an external control voltage whichchanges a maximum current of the light emission instruction signalgenerating unit. The maximum current of the light emission instructionsignal generating unit can be dynamically changed using an externalcontrol voltage. Thus, a shading correction operation during normaloperation of the system and slight adjustment of light intensity can beperformed.

A starting-up unit may be provided for allowing a start of operationwhen a light emission instruction current corresponding to the lightemission instruction signal reaches a predetermined current in powersupply starting. Similar to case of the above-mentioned starting-up unitfor the power source voltage, the initialization operation can beperformed with high accuracy, and also, the semiconductor laser can beprotected when the system is starting.

An adding current setting unit for setting a driving currentcorresponding to the light emission instruction signal and a combinedchanging unit for causing a maximum current of the light emissioninstruction signal generating unit and a maximum current of the addingcurrent setting unit to change together may be provided. Thereby, whenperforming the above-mentioned dynamic change of the maximum current ofthe light emission instruction signal generating unit for performing theshading correction operation, the maximum current of the additioncurrent setting unit is automatically changed together. Thereby, it ispossible that a light output waveform is close to an ideal rectangularwave, or is substantially a rectangular wave.

A starting-up unit for allowing a start of operation when a lightemission instruction current corresponding to the light emissioninstruction signal reaches a predetermined current in power supplystarting, an adding current setting unit for setting a driving currentcorresponding to the light emission instruction signal and a combinedchanging unit for causing a maximum current of the pulse widthmodulation and intensity modulation signal generating unit and a maximumcurrent of the adding current setting unit to change together may beprovided. When performing the above-mentioned dynamic change of themaximum current of the light emission instruction signal generating unitfor performing the shading correction operation, the maximum current ofthe addition current setting unit is automatically changed therewith. Itis possible that a light output waveform is close to an idealrectangular wave, or is substantially a rectangular wave.

An offset setting unit having an external connection device for settingan offset current which causes light output of the semiconductor laserto be a desired minimum value may be provided. In order to perform lightoutput control in the negative feedback loop in real time, light outputof the semiconductor laser should not be completely zero. Therefore, itis necessary to set the offset current for causing light output of thesemiconductor laser to be of the desired minimum value. By using theexternal device, the desired offset current can be easily set.

A combined setting unit having an external connection device forcombined setting of both a maximum current of the light emissioninstruction signal generating unit and an offset current which causeslight output of the semiconductor laser to be a minimum value may beprovided. Thereby, the offset current can be automatically setappropriately by only setting the maximum current of the light emissioninstruction signal generating unit using the external connection device.

An offset setting unit which has an external connection device forsetting an offset current which is used for causing light output of thesemiconductor laser to be a desired minimum value, and a combinedsetting unit having an external connection device for combined settingof both a maximum current of the light emission instruction signalgenerating unit and the offset current may be provided. The offsetcurrent can be automatically set appropriately by only setting themaximum current of the pulse width modulation and intensity modulationsignal generating unit using the external connection device. Also, usingthe offset setting unit, the offset current can be freely adjusted.

An abnormality detecting unit for detecting an abnormality in the signalof the output terminal of the semiconductor laser may be provided. Whenthe semiconductor laser is degraded but the degradation is not serious,the degradation may be detected and corrected through the negativefeedback loop and the current setting of the current driving unit.However, when the degradation is serious, a considerably large currentmay flow through the current driving unit. In such a case, theabove-mentioned abnormality detecting unit detects the abnormality, andthus serious trouble may be prevented from occurring.

The light emission instruction signal generating unit may include a basecurrent compensation unit for compensating base currents of transistorsconnected to a path of a reference current. Error currents due to basecurrents of transistors can be prevented from occurring, and transistorcharacteristic changes due to such error currents can be prevented.Thus, the current of the light emission instruction signal generatingunit can be provided with high accuracy.

An external connection device is provided for setting a current value ofthe light emission instruction signal generating unit. The lightemission instruction signal generating unit, which causes an absolutecurrent determined by the light reception device to flow, has a basecurrent compensation unit for compensating base currents of transistorsconnected to a path of a reference current. Thereby, the current of thelight emission instruction signal generating unit can be adjusted takingthe characteristics of the semiconductor laser and light receptiondevice into consideration. Thus, a desired light output can be obtained.Furthermore, error currents due to base currents of transistors can beprevented from occurring, and transistor characteristic changes due tosuch error currents can be prevented. Thus, the current of the lightemission instruction signal generating unit can be provided with highaccuracy.

According to the pulse width and intensity combined modulation methoddisclosed in Japanese Laid-Open Patent Application No.6-347852, imageforming is performed with a large number of tone levels. However, whenan image is formed in a laser printer or a copier, a large number oftone levels are not always needed for one dot. For example, when formingcharacter (letter) images, basically only two tone levels are needed.There are cases where higher writing density or higher writing speed isneeded rather than larger tone levels for one dot.

A detect pulse may be needed for obtaining a main-scan synchronizationsignal for determining a writing starting position in a main scandirection in a laser printer or the like. Further, a straight lineextending in a main scan line or a space extending in a main scan linemay be required. In these cases, according to the method disclosed inJapanese Laid-Open Patent Application No.6-347852, a semiconductor laseris driven with a pulse signal having a one-dot multi-tone output. It isconsidered that, in the above-mentioned cases of obtaining the detectpulse, main-scan direction extending line or space, it is not necessarythat the semiconductor laser is driven with one-dot multi-tone outputpulse signal, but with a simpler signal.

One object of the present invention is to provide a semiconductor lasercontrol system in which outputs suitable for particular image types canbe obtained.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier forming a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controls forwardcurrent of the semiconductor laser so that a light reception signalproportional to the light output of the semiconductor laser is equal tothe light emission instruction signal;

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current , the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop; and

output mode change-over means for selecting one ofclock-frequency-different output modes according to a frequencyselecting signal.

One of the clock-frequency-different output modes is selected accordingto a frequency selecting signal. Accordingly, for an image for which alarge number of tone levels is needed, the above-mentioned pulse widthand intensity level modulation method is effectively used. Thus, a highquality image having a large number of tone levels can be obtained.However, for an image for which a large number of tone levels are notneeded, the number of tone levels is reduced and a clock frequency isincreased. Thereby, pixel writing density in a main scan direction isincreased, and a high-quality image can be obtained.

A plurality of the pulse modulation and intensity modulation signalgenerating units may be provided, the number of the pulse modulation andintensity modulation signal generating units being equal to the numberof the different clock-frequency output modes, and the output modechange-over means may select one of the plurality of the pulsemodulation and intensity modulation signal generating units according tothe frequency selecting signal. Thereby, one of the differentclock-frequency output modes can be freely set and selected.

The pulse width modulation and intensity modulation signal generatingunit may comprise a plurality of modulation units for generating pulsewidth modulation data and intensity modulation data from input data, thenumber of the modulation units being equal to the number of thedifferent clock-frequency output modes, and one output unit whichperforms pulse width modulation and intensity modulation based on thepulse modulation data and intensity modulation data, and generates thelight emission instruction signal. The output mode change-over means mayselect one of the plurality of modulation units for the output unitaccording to the frequency selecting signal. Thereby, one of thedifferent clock-frequency output modes can be freely set and selected.Further, only one light emission instruction signal generating meansshould be provided. Thereby, it is possible to reduce the number ofnecessary components and miniaturization can be achieved.

The pulse width modulation and intensity modulation signal generatingunit may comprise pulse generating means for generating a plurality ofpulses having a frequency the same as the frequency of an input clocksignal and having different phases, the phase difference being a fixedphase difference, data converting means for converting input data intopulse width modulation data and intensity modulation data and pulsewidth modulation means for generating a plurality of pulses, which haveundergone pulse width modulation based on the pulse width modulationdata, from the pulses generated by the pulse generating means. Further,the output mode change-over means may cause the data converting means togenerate modulation data according to the frequency selecting signal andoutput it to the pulse width modulation means. Thereby, one of thedifferent clock-frequency output modes can be freely set and selected.The only necessary input terminals are those for the data convertingmeans, and the only one pulse width modulation means and the only onelight emission instruction signal generating means should be provided.Thus, the number of necessary components can be reduced andminiaturization can be achieved.

The clock frequencies of the output modes may comprise one which isequal to the clock frequency of the input clock signal and the otherwhich is double the clock frequency of the input clock signal, one ofwhich can be selected. Accordingly, for an image requiring a largenumber of tone levels, as one of the typical output modes, theabove-mentioned pulse width and intensity level modulation method iseffectively used. Thus, high quality images of a large number of tonelevels can be obtained. However, for an image not requiring a largenumber of tone levels, the number of tone levels is reduced and a clockfrequency is doubled. Thereby, pixel writing density in a main scandirection is doubled, and high-quality image can be obtained.

One of the different clock-frequency output modes may be an output modewhich is selected based on a forcible light emission instruction signalwhich comprises the frequency selecting signal. Thereby, all of theimage data is caused to be 0 and the forcible light emission instructionsignal is applied. Thus, it is possible that writing is performed with afrequency of the forcible light emission instruction signal, which hasno relation with the image data clock frequency. Such a mode of writingmay be effectively used as in the case of obtaining the above-mentioneddetect pulse.

One of the different clock-frequency output modes may be an output modewhich is selected based on a forcible light cessation signal whichcomprises the frequency selecting signal. For a portion for whichwriting is not required and without causing all of the image data to be0, mere use of the forcible light cessation signal is needed.

One object of the present invention is to provide a semiconductor lasercontrol system in which the current addition method which is performedfor effectively reducing an amount of control by the negative feedbackloop and the one-dot pulse width and intensity combined modulationmethod is performed with a miniaturized arrangement and with a reducedpower consumption, is performed with a large-scale-integration (LSI)integrated circuit, at high speed and with high accuracy, and outputssuitable for image types may be easily obtained.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal;

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop; and

output mode change-over means for selecting one of differentclock-frequency output modes according to a frequency selecting signal,

wherein the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier, the current driving unit and theoutput mode change-over means are formed as one chip of an integratedcircuit.

One of different clock-frequency output modes is selected according tothe frequency selecting signal. For an image requiring a large number oftone levels, as one of the typical output modes, the in-one-dot pulsewidth and intensity level modulation method is effectively used. Thus,high quality images having a large number of tone levels can beobtained. However, for an image not requiring a large number of tonelevels, the number of tone levels is reduced and a clock frequency isincreased. Thereby, pixel writing density in a main scan direction isincreased, and a high-quality image can be obtained. Furthermore, all ofa digital control system of the pulse width modulation and intensitymodulation signal generating unit, an analog driving system such as theerror amplifier and current driving unit and the output mode change-overunit are formed as one chip of an integrated circuit. Miniaturizationand energy saving can be achieved, and a high-speed, high accuracyoperation can be achieved.

A plurality of the pulse modulation and intensity modulation signalgenerating units may be provided, the number of the pulse modulation andintensity modulation signal generating units being equal to the numberof the different clock-frequency output modes, and the output modechange-over means may select one of the plurality of the pulsemodulation and intensity modulation signal generating units according tothe frequency selecting signal. Thereby, one of the differentclock-frequency output modes can be easily set and selected.

The pulse width modulation and intensity modulation signal generatingunit may comprise a plurality of modulation units for generating pulsewidth modulation data and intensity modulation data from input data, thenumber of the modulation units being equal to the number of thedifferent clock-frequency output modes, and one output unit whichperforms pulse width modulation and intensity modulation based on thepulse modulation data and intensity modulation data, and generates thelight emission instruction signal. The output mode change-over means mayselect one of the plurality of modulation units for the output unitaccording to the frequency selecting signal. Thereby, one of thedifferent clock-frequency output modes can be easily set and selected.Further, only one light emission instruction signal generating meansshould be provided. It is possible to reduce the number of necessarycomponents and further miniaturization of the integrated circuit can beachieved.

The pulse width modulation and intensity modulation signal generatingunit may comprise pulse generating means for generating a plurality ofpulses having a frequency the same as the frequency of an input clocksignal and having different phases, the phase difference being a fixedphase difference, data converting means for converting input data intopulse width modulation data and intensity modulation data and pulsewidth modulation means for generating a plurality of pulses, which haveundergone pulse width modulation based on the pulse width modulationdata, from the pulses generated by the pulse generating means. Theoutput mode change-over means may cause the data converting means togenerate modulation data according to the frequency selecting signal andoutput it to the pulse width modulation means. Thereby, one of thedifferent clock-frequency output modes can be easily set and selected.The only necessary input terminals are those for the data convertingmeans, and the only one pulse width modulation means and the only onelight emission instruction signal generating means should be provided.The number of necessary components can be reduced and furtherminiaturization of the integrated circuit can be achieved.

The clock frequencies of the output modes may comprise one which isequal to the clock frequency of the input clock signal and the otherwhich is double the clock frequency of the input clock signal, one ofwhich can be selected. For an image needing a large number of tonelevels, as one of the typical output modes, the above-mentioned pulsewidth and intensity level modulation method is effectively used. Thus,high quality images having a large number of tone levels can beobtained. However, for an image not needing a large number of tonelevels, the number of tone levels is reduced and a clock frequency isdoubled. Thereby, pixel writing density in a main scan direction isdoubled, and high-quality images can be obtained.

One of the different clock-frequency output modes may be an output modewhich is selected based on a forcible light emission instruction signalwhich comprises the frequency selecting signal. All of the image data iscaused to be 0 and the forcible light emission instruction signal isapplied. Thus, it is possible that writing is performed with a frequencyof the forcible light emission instruction signal, which has no relationwith the image data clock frequency. Such a mode of writing may beeffectively used in the case of obtaining the above-mentioned detectpulse.

One of the different clock-frequency output modes may be an output modewhich is selected based on a forcible light cessation signal whichcomprises the frequency selecting signal. For a portion where writing isnot required, without needing to cause all of the image data to be 0,mere use of the forcible light cessation signal is provided.

One object of the present invention is to provide a semiconductor lasercontrol system, an arrangement of the pulse width modulation andintensity modulation signal generating unit of which is appropriate forforming the system as one chip of an integrated circuit, the arrangementof the pulse width modulation and intensity modulation signal generatingunit being configured for selectively achieving both a large number oftone levels and a high-speed writing operation.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitcomprising pulse generating means, including an pulse oscillator, forgenerating a plurality of pulses having a frequency the same as thefrequency of an input clock signal and having different phases, thephase difference being a fixed phase difference, data converting meansfor converting input image data into pulse width modulation data andintensity modulation data and pulse width modulation means forgenerating a plurality of pulses, which have undergone pulse widthmodulation based on the pulse width modulation data, from the pulsesgenerated by said pulse generating means, said pulse width modulationand intensity modulation signal generating unit performing pulse widthmodulation and intensity modulation and generating a light emissioninstruction signal;

an error amplifier which forms a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal;

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop; and

wherein the pulse width modulation and intensity modulation signalgenerating unit, the error amplifier and the current driving unit areformed as one chip of an integrated circuit.

In the above-described arrangement, the pulse generating unit of thepulse width modulation and intensity modulation signal generating unitincludes the pulse oscillator. The pulse width modulation means shouldhave an arrangement merely for performing logic operations such aslogical multiplication and logical sum on a plurality of pulses outputfrom the pulse generating means. Accordingly, the arrangement of thepulse width modulation means can be easily formed and all of the systemcan be easily formed as one chip of an integrated circuit.

The pulse width modulation means may comprise n sets of two selectorsfor selecting pulses, based on mutually different pulse width modulationdata, from pulses generated by the pulse generating means, two logic ANDgates to which the outputs of the selectors and a non-inverted internalclock signal and an inverted internal clock signal are input, and alogic OR gate, to which the outputs of the logic AND gates are input,for outputting pulses. Thereby, a simple arrangement can be achieved.Such an arrangement is very suitable for a case in which freelyselective output modes include only an equal mode where the output modeclock frequency is equal to the input clock frequency and a double modewhere the output mode clock frequency is double of the input clockfrequency. This is because, in the double mode, data (pulse modulationdata) for each dot is a selecting signal for a respective one of the twoselectors. Thus, a simple arrangement can be achieved.

The data converting means, based on the image data and a frequencyselecting signal, may convert the image data into pulse width modulationdata and intensity modulation data. According to this arrangement, evenwhen the output mode is changed over according to the frequencyselecting signal, the frequency selecting signal is processed by thedata converting means. It is possible that the arrangement which followsthe data converting means is common for a plurality of selective outputmodes. The arrangement is appropriate for forming the system as one chipof an integrated circuit.

The data converting means, based on the image data, a frequencyselecting signal and a dot position control signal, may convert theimage data into pulse width modulation data and intensity modulationdata. In this arrangement, the dot position control signal is alsoprocessed by the data converting means. Accordingly, a dot phase(whether, for each dot, a waveform in which a pulse width starts fromthe left side or a waveform in which a pulse width starts from the rightside) can be freely selected for each dot, and high-quality images canbe obtained. The dot position control signal may comprise one bit.Thereby, the dot phase selection can be performed for each dot, or a dotconcentration type wave form (in which two adjacent dots are in contactwith one another) can be achieved with a minimum number of bits.

Means for generating the light emission instruction signal may comprisea digital to analog converter, a first current switch which allows ordoes not allow the non-inverted output current of the digital to analogconverter according to one of the pulses output from the pulse widthmodulation means and a second current switch which allows or does notallow the inverted output current of the digital to analog converteraccording to the other of the pulses output from the pulse widthmodulation means, the total of the output currents of the first andsecond current switches being used as the light emission instructionsignal.

The means for generating the light emission instruction signal can beformed in a simple arrangement, and all of the system can be easilyformed as one chip of an integrated circuit. Especially, the intensitymodulation data input to the digital to analog converter may be fixedduring one dot (that is, during one input clock period). Thus,high-speed writing can be achieved.

Means for generating the light emission instruction signal may comprisea digital to analog converter, a first current switch which allows ordoes not allow the non-inverted output current of the digital to analogconverter according to one of the pulses output from the pulse widthmodulation means and a second current switch which allows or does notallow the inverted output current of the digital to analog converter anda constant current equal to the least significant bit current of thedigital to analog converter according to the other of the pulses outputfrom the pulse width modulation means, the total of the output currentsof the first and second current switches being used as the lightemission instruction signal.

In this arrangement, a constant current equal to the least significantbit current is always caused to flow through the second current switch.Thereby, it is possible to increase the number of available tone levelswithout increasing the number of bits.

A third current switch may be provided for allowing or not allowing aconstant current based on a full on signal. Thereby, the full on stateoutput can also be provided.

The output mode clock frequencies may comprise one which is equal to theinput clock signal and the other which is double the input clock signal,one of which can be selected, and means of generating the light emissioninstruction signal may comprise two digital to analog converters whichconvert two intensity modulation data into currents, two first currentswitches which allow or do not allow the non-inverted output currents ofthe two digital to analog converters according to respective ones of thepulses output from the pulse width modulation means and two secondcurrent switches which allow or do not allow the inverted outputcurrents of the two digital to analog converters according to the otherrespective ones of the pulses output from the pulse width modulationmeans, the total of the output currents of the four current switchesbeing used as the light emission instruction signal.

In this arrangement, even when the double mode is selected, theintensity modulation data may be changed for a period which is the sameas the input clock period similar to the case of the intensity datamodulation data changing in the equal mode. Thereby, high-speed writingcan be achieved.

One object of the present invention is to provide a semiconductor lasercontrol system in which, even when output mode change-over is performedas described above, the number of input terminals is effectively reducedand a large number of tone levels is achieved and the system is formedas one chip of an integrated circuit.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitcomprising pulse generating means for generating a plurality of pulseshaving a frequency the same as the frequency of an input clock signaland having different phases, the phase difference being a fixed phasedifference, a fixed amount by the fixed amount, data converting meansfor converting input image data into pulse width modulation data andintensity modulation data and pulse width modulation means forgenerating a plurality of pulses, which have undergone pulse widthmodulation based on the pulse width modulation data, from the pulsesgenerated by the pulse generating means, the pulse width modulation andintensity modulation signal generating unit performing pulse widthmodulation and intensity modulation and generating a light emissioninstruction signal;

an error amplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of the semiconductor laser, the error amplifier controllingforward current of the semiconductor laser so that a light receptionsignal proportional to the light output of the semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through the semiconductorlaser as the forward current, the driving current being generated so asto control driving of the semiconductor laser with a current of thedifference or sum with the control current of the negative feedbackloop,

wherein:

the plurality of pulses output from the pulse width modulation meanshave a predetermined mutual relationship; and

the pulse width modulation and intensity modulation signal generatingunit, the error amplifier and the current driving unit are formed as onechip of an integrated circuit.

In the arrangement, the plurality of pulses output from the pulse widthmodulation means have a predetermined mutual relationship. By using themutual relationship, it is possible to cause some of modulation data tobe commonly used. Thereby, the number of required signals can bereduced. As a result, the number of input terminals and thecorresponding number circuit devices can be reduced. All of the systemcan be easily formed as one chip of an integrated circuit. Further,power consumption can be reduced.

One or a plurality of pulses of the plurality of pulses output from thepulse width modulation means may be always longer than the others by thefixed amount of phase difference. By using this mutual relationship,modulation data can be compressed and the number of required signals canbe reduced. Thus, a simple arrangement can be achieved.

Output mode clock frequencies may comprise one which is equal to theinput clock frequency and the other which is double the input clockfrequency, one which can be selected according to the frequencyselecting signal, 1/2 the bits of each image data being used for eachdot writing and the number of tone levels being 1/2 that of an equalfrequency mode case when a double frequency mode is selected.

Thereby, the number of necessary image data input terminals is equal inthe case of the equal mode and the case of the double mode. Accordingly,it is possible to commonly use the pulse width modulation means of thepulse width modulation and intensity modulation signal generating unit.The number of input terminals and the corresponding number of circuitdevices can be reduced. Thus, all of the system can be easily formed asone chip of an integrated circuit.

The data converting means may converts input image data into pulse widthmodulation data and intensity modulation data based on the image data, adot position control signal and a frequency selecting signal. Further,output mode clock frequencies may comprise one which is equal to theinput clock frequency and the other which is double the input clockfrequency, one which can be selected according to the frequencyselecting signal, each of two bits of input image data being used as thedot position control signal of a writing dot.

Accordingly, even in the double mode in which writing density has ahigher priority than the number of tone levels, whether a waveform inwhich a pulse width starts from the left side or a waveform in which apulse width starts from the right side can be freely selected for eachdot. The dot position control can be performed for each dot. As aresult, by alternately repeating a waveform in which a pulse widthstarts from the left side and a waveform in which a pulse width startsfrom the right side, the dot-concentration pulse modulation can beperformed. Thus, a desired type of output can be obtained.

The image data may comprise N-bit data series for representing 2^(N)output states, that is, a one-dot full turn on state, one-dot full turnoff state, and 2^(N-1) -1 middle levels, each of which has two levels ofdot phase states.

Accordingly, a predetermined number of tone levels can be obtained withone-bit-less data series. Therefore, the number of input terminals orthe input transfer rate required for obtaining the predetermined numberof tone levels can be reduced. Otherwise, a non one-bit-less data seriesis used, and the number of tone levels can be increased. As a result,this arrangement is advantageous when the available number of bits isrelatively small.

Only in the double frequency mode, the image data may comprise two setsof M-bit data series, each for representing 2^(M) output states, thatis, one-dot full turn on state, one-dot full turn off state, and 2^(M-1)-1 middle levels, each of which has two levels of dot phase states.

Accordingly, in the double mode in which the available number of bits issmall for each dot, the above-described advantage is effectively used,and, without increasing the number of signal lines, a predeterminednumber of tone levels can be obtained.

According to another aspect of the present invention, a semiconductorlaser control system, comprises:

a pulse width modulation and intensity modulation signal generating unitwhich, based on input data, performs pulse width modulation andintensity modulation and generates a light emission instruction signal;

an error amplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and

a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of thedifference or sum with the control current of said negative feedbackloop,

a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of said semiconductor laser;

a memory unit for storing a detection result of said differentialquantum efficiency detecting unit;

an adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection result storedin said memory unit; and

a timing generating unit,

wherein:

in initialization, said timing generating units generates a timingsignal sufficiently slower than the control speed of said erroramplifier, said differential quantum efficiency detecting unit detectsthe differential quantum efficiency of said semiconductor laser based onthe timing signal, said memory units storing a detection result at eachtiming, and the current corresponding to the light emission instructionsignal is set using the stored detection results;

said current driving unit comprises a high-speed voltage shifting unitin said error amplifier, includes a differential circuit for changingthe amount of voltage shift, and is provided in said negative feedbackloop.

output mode change-over means is provided for selecting one of differentclock-frequency output modes according to a frequency selecting signal;and

said pulse width modulation and intensity modulation signal generatingunit, said error amplifier, said current driving unit, said differentialquantum detecting unit, said memory unit, said adding current settingunit, said timing generating unit and said output mode change-over meansare formed as one chip of an integrated circuit by bipolar transistors.

In this arrangement, some features of the above-described aspects of thepresent invention are combined. Thereby, various functions can beperformed through the single semiconductor laser control system.

Other objects and further features of the present invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an I_(DA2) addition method with a current driving unit inthe related art;

FIGS. 2A, 2B show characteristics graphs in a case where Ps by I_(DA2)is not provided and in a case where Ps by I_(DA2) is provided;

FIG. 3 shows a block diagram of an example of a pulse width andintensity combined modulation method arrangement;

FIG. 4 shows typical views illustrating relationships between pulsewidth and intensity combined method light outputs and dot images;

FIGS. 5A, 5B show time charts illustrating how to generate pulse widthand intensity combined method waveforms;

FIG. 6 shows an input method in the related art;

FIG. 7 illustrates data waveform rising characteristics;

FIG. 8 is a characteristics graph showing operation current changecharacteristics due to temperature;

FIG. 9 is a characteristics graph showing operation current changecharacteristics due to elapsing time of use;

FIG. 10 shows a block diagram of a block arrangement of a semiconductorlaser control system in a first embodiment of the present inventionwhere the system is formed as one chip;

FIG. 11 shows a block diagram of a specific block arrangement of a lightemission instruction signal generating unit shown in FIG. 10;

FIG. 12A shows a block diagram of a specific block arrangement of a dataconverting unit and a pulse width modulation unit shown in FIG. 10;

FIG. 12B shows a block diagram resulting from partially rewriting acircuit diagram of the pulse width modulation unit;

FIG. 13 shows time charts illustrating a pulse width generating method;

FIG. 14 is a general block diagram showing an overall arrangement of asecond embodiment of the present invention;

FIG. 15 shows a circuit diagram of an example of an arrangement of anerror amplifier and a voltage shifting unit shown in FIG. 14;

FIG. 16 shows a circuit diagram of an example of an arrangement of alight emission instruction signal setting unit shown in FIG. 14;

FIG. 17 shows a circuit diagram of an example of an arrangement of alight emission instruction signal generating unit shown in FIG. 14;

FIG. 18 shows a circuit diagram of an example of an arrangement of apulse width modulation and intensity modulation signal generating unitshown in FIG. 14;

FIG. 19A shows a general arrangement of and around an input portion inthe second embodiment of the present invention;

FIG. 19B shows a general arrangement of a variant embodiment of aportion of the arrangement shown in FIG. 19A;

FIGS. 20A, 20B, 20C show time charts of a variant embodiment of an inputmethod;

FIG. 21 shows a general block diagram of a third embodiment of thepresent invention;

FIG. 22 shows a circuit diagram of an example of an arrangement of anoscillation circuit shown in FIG. 21;

FIG. 23A shows a circuit diagram of an example of an arrangement of afirst stage latch circuit of a latch circuit shown in FIG. 21;

FIG. 23B shows a circuit diagram of an example of an arrangement of alast stage latch circuit of the latch circuit shown in FIG. 21;

FIG. 24 shows time charts showing waveforms at respective portions;

FIG. 25 shows a circuit diagram of an example of an arrangement of adifferential quantum efficiency detecting unit shown in FIG. 21;

FIG. 26 shows a general block diagram of a variant embodiment of thethird embodiment;

FIG. 27 shows a general block diagram of a different variant embodimentof the third embodiment;

FIG. 28 shows a general block diagram of a fourth embodiment of thepresent invention;

FIGS. 29A, 29B, 29C show block diagrams of an example arrangements of alight emission instruction signal generating unit shown in FIG. 28;

FIG. 30 shows a block diagrams of example arrangement of the lightemission instruction signal generating unit and a switch unit shown inFIG. 28;

FIG. 31 shows a circuit diagram of an example arrangement of a powersource unit in a fifth embodiment;

FIG. 32 shows a circuit diagram of an example arrangement of astarting-up unit in the fifth embodiment;

FIGS. 33A, 33B, 33C show characteristic graphs when a current of a lightemission instruction signal generating unit and a current of a addingcurrent setting unit are increased together and when a current of thelight emission instruction signal generating unit and a current of theadding current setting unit are not increased together;

FIG. 34 shows a circuit diagram of a variant embodiment of the lightemission instruction signal generating unit;

FIG. 35 shows a general block diagram of a sixth embodiment of thepresent invention;

FIG. 36 shows a circuit diagram of an example arrangement of asemiconductor degradation detecting unit shown in FIG. 35;

FIG. 37 shows a block diagram of a variant embodiment of the sixthembodiment;

FIG. 38 shows a block diagram of a basic arrangement of seventh, eighth,ninth and tenth embodiments of the present invention;

FIGS. 39A, 39B, 39C show time charts showing cases where a writing clockfrequency is changed;

FIG. 40 shows examples of waveforms of light outputs and PW_(on),PW_(da) corresponding to a logic of the above-shown truth table;

FIG. 41 shows a general block diagram of the eighth embodiment of thepresent invention;

FIG. 42 shows a block diagram of a specific arrangement example of anarrangement shown in FIG. 41;

FIG. 43 shows a circuit diagram of an arrangement example of a latchcircuit shown in FIG. 42;

FIG. 44 shows a circuit diagram of an arrangement example for performinglogic operations of a portion of an arrangement shown in FIG. 42;

FIG. 45 shows a circuit diagram of an arrangement example for performinglogic operations of a portion of the arrangement shown in FIG. 42;

FIG. 46 shows a circuit diagram of a level shift circuit;

FIG. 47 shows a circuit diagram of a frequency selecting signalgenerating circuit;

FIG. 48 shows a circuit diagram of an arrangement example for obtainingintensity modulation signal;

FIG. 49 shows a circuit diagram of an arrangement example of amultiplexer in a pulse width modulation unit;

FIG. 50 shows a circuit diagram of an arrangement example of anotherportion in the pulse width modulation unit;

FIG. 51 shows a general block diagram of the ninth embodiment of thepresent invention;

FIG. 52 shows a general block diagram of the tenth embodiment of thepresent invention;

FIG. 53 shows a general block diagram of an eleventh embodiment of thepresent invention;

FIG. 54 shows a block diagram of an arrangement example of a pulsemodulation unit of the eleventh embodiment; and

FIG. 55 shows time charts showing an operation example of a double modecase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will now be described withreference to the accompanying drawings. A semiconductor laser controlsystem according to the present invention is applied as a controlsystem, including a negative feedback loop, for controlling light outputof a semiconductor laser which is used in optical writing in a laserprinter or the like. Further, the above-described pulse width andintensity combined modulation method (PWM+PM method) is used as a methodfor obtaining a multi-tone-level output within one dot.

For implementing the modulation method, a semiconductor laser controlsystem 100 in the first embodiment has a general arrangement such asthat shown in FIG. 3. In the arrangement, with reference to FIG. 3,image data and an input signal are input to pulse width generating unitand data modulation unit 11. The pulse width generating unit and datamodulation unit 11 outputs a light emission instruction signal to asemiconductor laser control unit and semiconductor laser driving unit12. A light reception device 2 is provided for a semiconductor laser 1,the light reception device 2 monitoring light output of thesemiconductor laser 1. The semiconductor laser 1 and light receptiondevice 2 are connected to the semiconductor laser control unit andsemiconductor driving unit 12. According to the input image data, thepulse width generating unit and data modulation unit 11 perform the PWMmethod, and the PM method for interpolating a change of a pulse width.

A basic concept of light output waveforms of the semiconductor laser 1is shown in FIG. 4. For the sake of simplicity of description, in anexample shown in FIG. 4, the number of pulse widths is three and thenumber of power levels is six. Thereby, a total of 18 tone levels areoutput. FIG. 4 typically shows light output waveforms in this case. Asshown in the figure, in this combined modulation method, basically thePWM method is used. Power modulation using a middle exposure range isperformed on a minimum pulse width. In order to obtain the light outputwaveforms, for example, as shown in FIGS. 5A and 5B, either a pulse 1having a pulse width of T and a pulse 2 having a pulse width of T+δT (δTbeing the minimum pulse width) or a pulse 3 having a pulse width of Tand a pulse 4 having a pulse width of δT are generated. For the pulse ofthe pulse width of T, each of the bits is of the H level, while, for thepulse of the pulse width δT (in the case of the pulse 2, during the timeδT only the pulse 2 is at the high level), respective bits may be ofeither the H level or the L level according to input image data.Thereby, the light output waveforms shown in FIG. 4 and FIGS. 5A, 5B canbe provided. In the example shown in FIG. 5A, a pulse width starts fromthe left side, and, in the example shown in FIG. 5B, a pulse widthstarts from the right side.

With reference to FIG. 10, a detailed block arrangement of thesemiconductor laser control system in the first embodiment will now bedescribed. The semiconductor laser control unit and semiconductor laserdriving unit 12 includes a negative feedback loop 106 and aconstant-current source 107 which forms a driving unit. The negativefeedback loop 106 includes an error amplifier 108 which is connectedwith the semiconductor laser 1 and light reception device 2 to form aloop and forms an error amplifying unit. The negative feedback loop 106monitors light output of the semiconductor laser 1 through the lightreception unit 2, and always controls a forward current of thesemiconductor laser 1 so that the monitored light output is equal to alight emission instruction signal (I_(DA1)) which is generated by thesemiconductor laser control unit and semiconductor laser driving unit12. The constant-current source 107 functions so as to cause a drivingcurrent I_(DA2) according to a light emission instruction signal(V_(DA2)) to flow through the semiconductor laser 1 in its forwarddirection. In the semiconductor laser control unit and semiconductorlaser driving unit 12, light output of the semiconductor laser 1 iscontrolled basically by a current of a sum (or a difference) of acontrol current of the negative feedback loop 106 and the drivingcurrent of the constant-current source 107.

In the circuit configuration, light output corresponding to a currentthrough which the constant-current source 107 directly drives thesemiconductor laser 1 is referred to as P_(S), the step responsecharacteristics of the light output of the semiconductor laser can beapproximated, as follows:

    P.sub.out =P.sub.0 +(P.sub.S -P.sub.0){1-exp(-2πf.sub.0 t)}.

When P_(S) ≈P₀, the light output of the semiconductor laser 1immediately becomes equal to P₀. Therefore, f₀ may have a relativelysmall value in comparison to the case where there is only the negativefeedback loop 106. In a practical case, f₀ may have a value ofapproximately 40 (MHz). Such a cutoff frequency f₀ can be easilyobtained. FIG. 2A shows how the light output changes only through thenegative feedback loop 106 (control unit). FIG. 2B shows how the lightoutput changes in the case where the constant current I_(DA2) is addedby the constant-current source 107. As shown in the figures, the lightoutput waveform shown in FIG. 2B is similar to that of a rectangularwave.

In the above-described semiconductor laser control system 100 in thefirst embodiment, the pulse width generating unit and data modulationunit 11 and the semiconductor laser control unit and semiconductor laserdriving unit 12 are formed as one chip of an integrated circuit 13 bybipolar transistors. The negative feedback loop 106 portion includingthe error amplifier 108 can be formed as an integrated circuit, notshown, but as shown in FIG. 2 of Japanese Laid-Open Patent ApplicationNo.5-67833 (U.S. Pat. No. 5,237,579), for example, using a known bipolartransistor circuit. Also, the constant-current source 107 portion can beformed as an integrated circuit, not shown, but as shown in FIGS. 13 and17 of Japanese Laid-Open Patent Application No.5-67833 (U.S. Pat. No.5,237,579), for example, using a known bipolar transistor circuit.

In the integrated circuit 13, a more detailed arrangement and functionsof the pulse width generating unit and data modulation unit 11 portionwill now be described. In the first embodiment, pulse width modulationis performed with three bits (that is, eight levels), and intensitymodulation is performed with five bits (that is, 32 levels). In total,it is possible to output 8-bit tone levels (that is, 256 levels). Thepulse width generating unit and data modulation unit 11 include a pulsewidth modulation intensity modulation signal generating unit 111 andlight emission instruction signal generating unit 112.

The light emission instruction signal generating unit 112 includes, asshown in FIG. 11, a digital to analog converter (DAC) 113 whichgenerates the currents I_(DA) and I_(DA) according to intensitymodulation data PMDATA (PMD), a differential switch 114A which allows ordoes not allow the current I_(DA) to flow therethrough according to apulse 1 (PW_(on)), a differential switch 114B which allows or does notallow the current I_(DA) to flow therethrough according to a pulse 2(PW_(da)), current to voltage converters (IVC) 115A, 115B which converta currents flowing through the switches 114A, 114B into voltages,respectively. It is noted that I_(DA) +I_(DA) =I_(full). The value ofI_(full) is the value of I_(DA) when all PMDATA is ON, and is themaximum current value of the light emission instruction signal. Thedifferential switches 114A, 114B function so that, when each of thepulse 1 and pulse 2 is the H level, I_(DA1) =I_(full). When the pulse 1is the L level and the pulse 2 is the H level, I_(DA1) =I_(DA). Wheneach of the pulse 1 and pulse 2 is the L level, I_(DA1) =0. When each ofthe pulse 1 and pulse 2 is the H level, I_(DA1) =I_(full), withoutdepending on the value of I_(DA) (that is, without depending on PMDATA).Therefore, the intensity modulation data PMDATA can be fixed during onepixel clock period. This is advantageous in achieving a high-speedoperation of the semiconductor. laser control system. Each of suchdifferential switches as the switches 114A, 114B can be formed as aresult of differential connection of a pair of bipolar transistors. Thecurrent to voltage converters 115A and 115B provide the light emissioninstruction signal V_(DA2) of two voltages to the constant-currentsource 107 shown in FIG. 10. The constant-current source 107 generatesthe current I_(DA2) according to the difference between the two voltagesof the light emission instruction signal V_(DA2). Each of the current tovoltage converters 115A, 115B can also be formed of a bipolartransistor, a base of which is connected to the ground, for example.Consequently, the light emission instruction signal generating unit 112itself can be easily formed to be an integrated circuit by bipolartransistors.

The pulse width modulation intensity modulation signal generating unit111 of the pulse width generating unit and data modulation unit 11includes, for example, a data converting unit 116 acting as dataconverting means, a pulse width modulation unit 117 acting as pulsewidth modulation means and a pulse generating oscillator 118 which is ofa PLL configuration. The pulse generating oscillator 118 generates aninternal clock signal X₀ which is in synchronization with an input clocksignal, and a plurality of pulse signals X₁, X₂, . . . , X_(k). Theplurality of pulse signals X₁, X₂, . . . , X_(k) have the same frequencyas that of the internal clock signal, and have different phases fromeach other, the phase difference being a fixed phase difference. Thus, apulse phase shifts by the fixed phase difference between the differentphases (the fixed phase difference corresponding to the minimum pulsewidth δT), from X₀ to X₇, as shown in FIG. 13. When pulse widthmodulation is of 8 levels, k=7, and the uniform phase shift is1/8·T_(CK), as shown in FIG. 13, where T_(CK) represents the period ofthe input clock signal. As shown in the figure, the signals X₄, X₅, X₆,X₇ are the inverted signals of the signals X₀, X₁, X₂, X₃, respectively.Any of the 8 pulse signals X₀, X₁, X₂, . . . , X₇ can be a pulse signalwhich is actually in synchronization with the input clock signal. In theexample shown in FIG. 13, the signal X₆ is actually in synchronizationwith the input clock signal. Actually, the internal clock signal X₀ isdelayed 1/4 period from the input clock signal, as shown in the figure.As shown in FIG. 12A (FIG. 12B will be described later), the eight pulsesignals X₀, X₁, X₂, . . . , X₇ are provided to the pulse widthmodulation unit 117. The data converting unit 116 converts input imagedata D₀, D₁, D₂, . . . , D₇ into pulse width modulation data PWMDATA andintensity modulation data PMDATA. According to the pulse widthmodulation data PWDATA provided by the data converting unit 116, thepulse width modulation unit 117 generates two pulse signals, pulsesPW_(on) (pulse 1) and pulses PW_(da) (pulse 2) from the output X₀, X₁,X₂, . . . , X₇ of the pulse generating oscillator 118.

A logic operation for obtaining a light output waveform, started fromthe left side, such as that shown in FIG. 5A is expressed by thefollowing equations (1) and (2): ##EQU1##

D_(n1), D_(n2), D_(m1), D_(m1), D_(n1) ', D_(n2) ', D_(m1) ', D_(m1) ',are the pulse width modulation data PWMDATA. When the more significantrank three bits, that is, D₇, D₆, D₅ of the image data D₇ (MSB), D₆, D₅,. . . , D₀ (LSB) are data for the pulse width modulation, theabove-mentioned pulse width modulation data PWMDATA is expressed by thefollowing equation (3): ##EQU2##

In order to perform the logic operation, the data converting unit 116and pulse width modulation unit 117 are configured as shown in FIG. 12A.The data converting unit 116 includes logic units 121, 122, 123 and 124which converts the image data D₀, D₁, D₂, . . . , D₇ into the pulsewidth modulation data D_(ni), D_(ni) ', D_(mj), D_(mj) ' according tothe above-mentioned equation (3). The data converting unit 116 furtherincludes a logic unit 125 which outputs the data of the less significantfive bits, that is, D₄, D₃, D₂, D₁ and D₀ of the image data D₇, D₆, D₅,. . . , D₀ as the intensity modulation data D_(pk) (PMD) as it is. Theselogic units 121, 122, 123 and 124 include means for holding modulationdata such as latches or the like. The pulse width modulation unit 117includes multiplexers 126, 127, 128, 129 (acting as selectors), each ofwhich selects one of the pulse signals X₀, X₁, X₂, . . . , X₇, using thepulse width modulation data D_(ni), D_(ni) ', D_(mj), D_(mj) '. Theseoperations of the multiplexers 126, 127, 128 and 129 are equivalent tothe logical operation of the above-mentioned equations (2). The pulsewidth modulation unit 117 further includes AND gates 30a, 30b, 30c, 30dand OR gates 30e, 30f which perform the logic operation shown in theabove-mentioned equations (1). The OR gate 30e outputs the pulsesPW_(da) and the OR gate outputs the pulses PW_(on). These dataconverting unit 116 and the pulse width modulation unit 117 which mainlyperforms logic operation can also be formed to be an integrated circuitby bipolar transistors.

As a result, in the first embodiment, the entirety of the pulse widthgenerating unit and data modulation unit 11 and the semiconductor lasercontrol unit and semiconductor unit driving unit 12 can be formed as onechip of an integrated circuit 13 by bipolar transistors. Thereby, indriving and controlling the semiconductor laser in pulse width andintensity combined modulation method within one dot using thecombination of the negative feedback loop 106 and the addition currentvalue control system (such as that, advantage of which was describedwith reference to FIGS. 2A and 2B), it is possible to achieveminiaturization of the system and energy saving. Because all thenecessary processing is performed within the one chip of the integratedcircuit 13, the processing can be performed at high speed with highaccuracy. By forming the one chip of the integrated circuit 13 usingbipolar transistors, it is easy to form an analog driving amplifier suchas the error amplifier 108 and constant-current source 107, input levelsthereof can be freely set, and it is possible to set the input levels tolow levels. Thereby, it is possible to improve functions of laserprinter or the like.

In the first embodiment, the pulse width generating unit and datamodulation unit 11 and semiconductor laser control unit andsemiconductor laser driving unit 12 are formed as one chip of anintegrated circuit using only bipolar transistors. However, it is alsopossible to form them to be one chip of an integrated circuit using onlyC-MOS transistors. It is also possible to form them to be one chip of anintegrated circuit using both bipolar transistors and C-MOS transistorsas a combined circuit. When one chip of an integrated circuit is formedusing C-MOS transistors, it is easy to form the pulse width generatingunit and data modulation unit 11 portion which is a digital controlsystem. Further, it is possible to obtain a highly-integrated integratedcircuit. When one chip of an integrated circuit is formed using bothbipolar transistors and C-MOS transistors as a combined circuit, analogdriving amplifiers such as the error amplifier 108 and constant-currentsource 107 can be easily formed using bipolar transistors and the pulsewidth generating unit and data modulation unit 11 which is a digitalcontrol system can be easily formed using C-MOS transistors. Thus,circuit designing can be more easily performed.

A second embodiment of the present invention will now be described withreference to the drawings. In the second embodiment, the above-describedpulse width and intensity combined modulation method and combination ofthe negative feedback loop 106 and the addition current value controlsystem, which combination effectively reduces the load of the negativefeedback loop, are used. The same reference numerals as those shown inFIGS. 1 and 3 are given to identical components/parts in the secondembodiment.

A semiconductor laser control system 213 in the second embodiment, inoutline, includes, as shown in FIG. 3, the pulse width generating unitand data modulation unit 11 (pulse width modulation and intensitymodulation signal generating unit) and semiconductor laser control unitand semiconductor laser driving unit 12 (semiconductor control anddriving unit).

FIG. 14 shows an example of a detailed arrangement of the semiconductorlaser control system 213 in the second embodiment. In the secondembodiment, almost all components/parts of the pulse width modulationand intensity modulation signal generating unit 11 and the semiconductorcontrol and driving unit 12 are formed as one chip of an integratedcircuit 220. In more detail, as examples of partial circuit arrangementswill be shown, almost all of the components/parts are formed as one chipby bipolar transistors.

The semiconductor laser control and driving unit 12 will now bedescribed. The negative feedback loop 3 includes a light emissioninstruction signal setting unit 221, a light emission instruction signalgenerating unit 222, an error amplifier 223 (corresponding to theinverting amplifier 6), a current driving unit 224, the semiconductorlaser 1 and the light reception device 2. The light emission instructionsignal generating unit 222 includes a first light emission instructionsignal generating unit 222a and a second light emission instructionsignal generating unit 222b. A current which is generated by the firstlight emission instruction signal generating unit 222a according tomodulated data is compared with a monitor current which is output by thelight reception device 2 and is in proportion to light output of thesemiconductor laser 1. An error amount resulting from the comparison isconverted into a forward current of the semiconductor laser 1 throughthe error amplifier 223 and current driving unit 224. When the monitorcurrent is larger than the current from the first light emissioninstruction signal generating unit 222a, a forward current of thesemiconductor laser 1 decreases, and, when the monitor current issmaller than the current from the first light emission instructionsignal generating unit 222a, a forward current of the semiconductorlaser 1 increases. Thus, the negative feedback loop 3 is configured.Generally speaking, the differential quantum efficiency of thesemiconductor laser 1 and light-to-electricity conversion lightreception sensitivity of the light reception device 2 may be differentdepending on particular products. It is necessary to set a current valueaccording to particular characteristics. The light emission instructionsignal setting unit 221 sets a value of the current I_(DA1), that is, avalue of a monitor current I_(PD) of the light reception device 2 whenconsidering a steady state, according to an external current settingsignal so that the semiconductor laser 1 may provide desired lightoutput. It is possible to set the current value so that characteristicsvariation depending on particular produces is considered and thesemiconductor laser 1 may always provide a desired light output.

The current driving unit 224 is formed to be, for example, adifferential switch arrangement, and acts as a high-speed voltageshifting unit 225 which instantaneously shifts output of the erroramplifier 223 by a desired electric potential. This voltage shifting ofthe high-speed voltage shifting unit 225 instantaneously effects acorresponding change in a forward current of the semiconductor laser 1.Thus, high-speed modulation of the semiconductor laser 1 can bepossible. The high-speed voltage shifting unit 225 which functions asthe current driving unit 224 is included in a control system of thenegative feedback loop 3 and has an output portion which is the same asan output portion of the negative feedback loop 3. In forming theintegrated circuit 220, it is advantageous that the number ofcomponents/parts can be effectively reduced and power consumption of thecircuit can also be effectively reduced.

FIG. 15 shows an example of a circuit arrangement using bipolartransistors of the error amplifier 223 and high-speed voltage shiftingunit 225. The monitor current I_(PD), which flows through the lightreception device 2 in proportion to light output of the semiconductorlaser 1, flows, via a PD terminal, to the base of a transistor Q₁ of thelight emission instruction signal generating unit 222 (first lightemission instruction signal generating unit 222a). A digital to analogconverter, which will be described later, in the light emissioninstruction signal generating unit 22 converts input data into a currentI_(DA1), and the current I_(DA1) flows out from the base of thetransistor Q₁. A result of comparison between the currents I_(PD) andI_(DA1) is detected at the base of the transistor Q₁. A result of thedetection is input to a differential amplifier 241 including transistorsQ₂, Q₃. Output of the differential amplifier 241 is input to a base of atransistor Q₈ which is the same as the transistor 7 shown in FIG. 14.The transistor Q₈ causes a forward current to flow through thesemiconductor laser 1, and the current also flows through a resistor R₁which is the same as the resistor Re shown in FIG. 14. Thus, thenegative feedback loop 3 is formed. Between the differential amplifier241 to the terminal LD of the semiconductor laser 1, a differentialswitch 242 of a differential circuit including transistors Q₄, Q₅ and aresistor R₄ is connected. The high-speed voltage shifting unit 225 isformed, which instantaneously shifts the voltage of a signal transmittedfrom the differential switch 241 to the transistor Q₈. This voltageshifting instantaneously effects a corresponding change of a forwardcurrent of the semiconductor laser 1 via an emitter follower 243including transistors Q₆, Q₇ and Q₈. In the embodiment, the drivingtransistor 7 and resistor Re which finally drive the semiconductor laser1 are externally connected to the integrated circuit 220. It isnecessary to provide a current of several tens to several hundredmilli-amperes to flow to the semiconductor laser 1. However, in thecircuit arrangement of the present invention, within the semiconductorlaser control and driving unit 12, even at an output portion which isdirectly connected to a driving portion (the driving transistor 7), themaximum current value is merely several milli-amperes. Thereby, powerconsumption is reduced and forming the circuit as an integrated circuit(development of LSI circuit) is easy. In the circuit shown in FIG. 15,an amount of the voltage shift of the current driving unit 224 isdetermined by resistors R₂, R₃, a transistor Q₉ and so forth. However,as mentioned above, the differential quantum efficiency of thesemiconductor laser 1 may vary according to a particular product, andthe efficiency may be degraded according to time elapsing. In order tosolve this problem, a differential quantum efficiency detecting unit 232detects the differential quantum efficiency of the semiconductor laser1, and, based on a result of the detection, the amount of the voltageshift is set. Thereby, improved light output resulting from light outputPs being added such as that shown in FIG. 2B can be obtained. Thedifferential amplifier 241 including the transistors Q₂, Q₃ provides itsoutput as a voltage drop from a power source voltage Vcc through aresistor R₄. The power source voltage Vcc may vary. However, because thenegative feedback loop 3 controls light output of the semiconductorlaser in real time, variation of the power source voltage Vcc can beautomatically processed in the control, and does not finally adverselyaffect the control. Further, as a result of the detection at the PDterminal (the base of the transistor Q₁ in the first light emissioninstruction signal generating unit 22a) being input to the differentialamplifier 241, a feedback is performed through transistors Q₁₀, Q₁₁ anda resistor R₆, the voltage gain of the differential amplifier 241 isdetermined by resistance values of resistors R₅, R₆, and the gain isreduced to a small level. Thereby, the cutoff frequency of thedifferential amplifier 241 is increased and control speed is improved.The resistors R₅, R₆ may be externally connected components. By changingthe resistance values of the resistors R₅, R₆, control speed of thecontrol system (negative feedback loop 3) can be changed.

In FIG. 14, a timing generating unit 231, the differential quantumefficiency detecting unit 232, a memory unit 233 and an adding currentsetting unit 234 form a block which detects the differential quantumefficiency of the semiconductor laser 1 and sets the amount of thevoltage shift. Thereby, the timing generating unit 231 generates atiming signal, timing of which is sufficiently slower than the controlspeed of the error amplifier 223. In the timing, the differentialquantum efficiency of the semiconductor laser 1 is detected by thedifferential quantum efficiency detecting unit 232. A result of thedetection is stored in the memory unit 233. According to the data storedin the memory unit 233, a current value of the adding current settingunit 234 is set. These operations are performed as an initializingoperation only in a predetermined initializing time such as a powersupply starting time or a resetting time (a time when no light output isprovided by the semiconductor laser 1). During a normal operation time,the current value of the adding current setting unit 234 is maintained.A starting-up unit 235, which is connected with the timing generatingunit 231 in the integrated circuit 220, starts the initializingoperation of the integrated circuit 220.

Examples of circuit arrangements of the light emission instructionsignal setting unit 221 and light emission instruction signal generatingunit 222 using bipolar transistors will be described with reference toFIGS. 16 and 17.

The light emission instruction signal setting unit 221 performs settingof a reference current of the light emission instruction signalgenerating unit 222, setting of a reference current of the addingcurrent setting unit 234, a base current compensation of the current ofthe light emission instruction signal generating unit 222, andadjusting, with an external signal, the reference current of the lightemission instruction signal generating unit 222 and the referencecurrent of the adding current setting unit 234 together. With referenceto FIG. 16, these functions will now be described.

The setting of a reference current of the light emission instructionsignal generating unit 222 is performed by an emitter voltage of atransistor Q₇₁ and a resistor R₄₁. The current of the light emissioninstruction signal generating unit 222 I_(DA1) is equal to the monitorcurrent I_(PD) of the light reception device 2 when considering a steadystate. Therefore, the current should be one which is not affected by atemperature change occurring inside the integrated circuit 220 (LSIcircuit). In other words, the emitter voltage of the transistor Q₇₁should be a stable voltage and the resistor R₄₁ should be a resistorhaving an absolute accuracy. For this purpose, the emitter voltage ofthe transistor Q₇₁ is provided as a result of a voltage of a VREF11terminal, which voltage is a stable voltage generated by a power sourceunit, being transmitted via a voltage follower 244 including transistorsQ₇₂, Q₇₃, Q₇₄ and Q₇₅. In FIG. 16, the resistor R₄₁ is indicated as ifthe resistor is an internal resistor of the integrated circuit. However,actually, the resistor R₄₁ is an external resistor or an externalvariable resistor which has absolute accuracy and superior temperaturecharacteristics, and is connected to the integrated circuit via anexternal terminal VR. By changing the resistance value of the resistorR₄₁, adjustment is possible so that influence of variation in thecharacteristics of the semiconductor laser 1 and light reception device2 is removed and a desired light output can be obtained.

The setting of a reference current of the adding current setting unit234 is performed by an emitter voltage of a transistor Q₇₈ and aresistor R₄₂, and the set reference current is output to the addingcurrent setting unit 234 via an IDA2SET terminal. The emitter voltage ofthe transistor Q₇₈ is approximately equal to the emitter voltage of thetransistor Q₇₁ as a result of the emitter voltage of the transistor Q₇₁being transmitted to the emitter of the transistor Q₇₈ via thetransistor Q₇₁, transistors Q₇₆, Q₇₇ and the transistor Q₇₈.

The base current compensation of the current of the light emissioninstruction signal generating unit 222 is performed by a base current ofthe transistor Q₇₇. As described above, the current I_(DA1) of the lightemission instruction signal generating unit 222 corresponds to thecurrent I_(PD) which is an absolute current determined by the externallight reception device 2. A reference current thereof is determined bythe emitter voltage of the transistor Q₇₁ and the resistor R₄₁ and is anabsolute current. However, after the reference current passes through acurrent mirror circuit 245, the current passes through some transistorsand the current I_(DA1) flows out from the terminal PD. When passingthrough some transistors, base current errors occur. Such base currenterrors occur in each bit of a 5-bit digital to analog converter (b0, b1,b2, b3, b4). The base current of the transistor Q₇₇, which isappropriately adjusted, compensates the base current errors. Thereby, itis possible to easily perform base current compensation even whencharacteristics of the transistors vary. The electronic circuit shown inFIG. 17 corresponds to the block arrangement shown in FIG. 11. Withreference to FIG. 11, the current I_(DA) passes through the transistorsof DAC 113, the transistor of the switch 114B and the transistor of theIVC 115B. The above-described base current compensation compensates basecurrent errors occurring while the current I_(DA) is passing throughthese transistors.

The adjusting, with an external signal, of the reference current of thelight emission instruction signal generating unit 222 and the referencecurrent of the adding current setting unit 234 together will now bedescribed. As described above, the setting of the reference current ofthe light emission instruction signals generating unit 222 and thesetting of the reference current of the adding current setting unit 234are determined by the emitter voltage of the transistor Q₇₁ and theresistor R₄₁. Further, the emitter voltage of the transistor R₇₁ isprovided from the VREF11 terminal voltage through the voltage follower244 including the transistors Q₇₂, Q₇₃, Q₇₄, Q₇₅. In parallel with theinput from the VREF11 terminal, a control voltage (external voltage) isinput from a VCONT terminal through resistors R₄₃, R₄₄ and a transistorQ₇₉. By changing the control voltage of the VCONT terminal, the emittervoltage of the transistor Q₇₁ is changed. Thus, the reference current ofthe light emission instruction signal generating unit 222 and thereference current of the adding current setting unit 234 areincreased/decreased together. Thereby, light output change by thenegative feedback loop and light output change by the addition currentvalue control system can be performed together. Therefore, light outputchange performed for shading correction can be properly performed whilemaintaining a waveform, similar to a rectangular wave, such as thatshown in FIG. 2B.

With reference to FIG. 17, the light emission instruction signalgenerating unit 222 will now described. The light emission instructionsignal generating unit 222 includes the 5-bit digital to analogconverter (b0, b1, b2, b3, b4) and an electric current addition drivingunit. 5 bits of digital data which is converted into analog data throughthe digital to analog converter of the light emission instruction signalgenerating unit 222 is PMDATA (intensity modulation signal) which isprovided by the PWM & PM signal generating unit 11 shown in FIG. 14.However, when it is necessary to set light output with higher accuracy,it is possible to increase the number of bits of the digital to analogconverter. When pulse width modulation is mainly used, it is possible toreduce the number of bits of the digital to analog converter. In theembodiment, the digital to analog converter includes a combination ofcurrent mirror circuits and a resistor ladder. However, anotherarrangement of the digital to analog converter having equivalentfunctions may be possible.

The current addition driving unit of the light emission instructionsignal generating unit 22 detects the current I_(DA1) and the invertedcurrent as emitter voltages of transistors Q₈₁, Q₈₂, and inputs them tobases of the transistors Q₄, Q₅, through emitter followers oftransistors Q₈₃, Q₈₄, respectively. The emitter voltages of thetransistors Q₈₁, Q₈₂ are voltages reflect the current values of I_(DA1)Thereby, in the differential switch 242 including the transistor Q₄, Q₅shown in FIG. 15, when the digital to analog converter is of the 5-bitdigital to analog converter, not merely ON/OFF two level output but5-bit multi level current driving output can be provided at high speed.

FIG. 18 shows an arrangement of the pulse width modulation and intensitymodulation signal generating unit 11. In the embodiment, 3-bit (that is,8 levels) pulse width modulation and 5-bit (that is, 32 levels)intensity modulation are combined, and an 8-bit tone (256 levels) isoutput for each dot. The pulse width modulation and intensity modulationsignal generating unit 11 includes, for example, a data converter 116 ,a pulse width modulation unit 117 and a pulse generating oscillator 118,which may be similar to the data converter 116 , pulse width modulationunit 117 and pulse generating oscillator 118 of the first embodimentshown in FIG. 10. Duplicated descriptions of arrangements and functionsthereof will be omitted.

A portion in the integrated circuit 220 to which the image data D₀, D₁,D₂, . . . , D₇ is input will be described with reference to FIGS. 19Aand FIG. 19B. The data converter 116 in the integrated circuit 220 ofbipolar transistors includes an ECL (Emitter Coupled Logic) circuit 271provided as shown in FIG. 19A at an input portion thereof. The ECLcircuit 271 includes a pair of transistors Qa, Qb, emitters of which areconnected as an differential arrangement, and a constant-current source272 connected to the emitters. The ECL circuit 271 establishes a logicoperation, that is, positively provides logic data when a differencebetween base voltages Va, Vb of the transistors Qa, Qb is approximately200 mV. Therefore, when a voltage Vb is fixed, for example, a voltage Vashould be larger than the voltage Vb by 200 mV , that is, Va≧Vb+200 (mV)or smaller than the voltage Vb by 200 mV, that is, Va≦Vb-200 (mV). Evenwhen considering fluctuation of the voltages, the sufficient requiredvoltage difference is 250 mV. Consequently, a sufficient required amountof swinging voltage Va with respect to the fixed voltage Vb is 500 mV.Normally, image data has a voltage swing of 0 to 5 volts. However,because the above-described ECL circuit 271, which merely required the500 mV amount of voltage swing, is inserted at the input portion of theintegrated circuit 220, the swing amount of the image data may bereduced to 0 to 500 mV. For this purpose, specifically, as show in FIG.19A, a resistor Ra is provided in a transmission line such as harness 73into which image data having an amount of voltage swing 0 to 5 volts isinput, and a resistor Rb is provided between a power source terminal of5 volts and the transmission line, where a resistance value ratio ofRa:Rb≈9:1, for example, Ra=1.5 (kΩ), Rb=165 (Ω). Such an electriccircuit forms an impedance matching circuit 274.

Through such an arrangement, when image data having a voltage swing of 0to 5 volts is input to the resistor Ra, the voltage at a point (inputpoint), at which the transmission line and the resistor Rb areconnected, swings between 4.5 to 5 volts. Thus, the swing amount at theconnected point is 0 to 500 mV, and image data is input to the ECLcircuit 271 of the integrated circuit 220 after the amount of voltageswing is reduced to 1/10 of the original. When considering τ=CR=C·(V/I)for a time constant τ which was described above with reference to FIG.7, it is possible to apparently reduce the time constant τ as a resultof reducing an amount of input voltage swing and maintaining a currentto be the same. That is, it is possible to achieve high-speed datatransfer. High-speed data transfer is possible up to approximately 70 to80 MHz. It is possible to reduce a driving amount by inputting imagedata after reducing the amount of voltage swing, and it is possible toreduce to 1/100 an original amount in energy. Thereby, energy saving canbe achieved, and also, such an arrangement is advantageous in taking EMImeasures. The input portion is formed as the impedance matching circuit274, and reflection of input data is not likely to occur.

When the input portion is formed as the impedance matching circuit 274,it is also possible that the resistor Rb is connected to the ground sideas shown in FIG. 19B.

Further, assuming original image data to be a pulse of a voltage V asshown in FIG. 20A, it is possible to divide it into a combination of apositive logic data of a voltage V/2 such as that shown in FIG. 20B andnegative (inverted) logic data of voltage V/2 such as that shown in FIG.20C. These two logic types of image data are input in parallel to theintegrated circuit 220 via two transmission lines. Specifically, thepositive logic data such as that shown in FIG. 10B is input to the baseof the transistor Qa, and the negative logic data such as that shown inFIG. 10C is input to the base of the transistor Qb. Thus, differentialoutput of the two types of input data is provided by the ECL circuit271. In this case, a transmission line circuit and an input transistorwith a constant-current source the same as those connected to the baseof the transistor Qa are connected to the base of the transistor Qb. Insuch a case, because the positive logic data and negative logic data arecombined as mentioned above, a necessary swing amount at the input pointfor each type logic data is merely 250 mV.

In such an input arrangement, because energy is in proportion to(voltage)², the necessary energy is 1/4 the necessary energy in the caseof FIG. 19A using the single input image data shown in FIG. 20A.Further, if image data includes noise, because the noise affects to boththe positive logic data and negative logic data the same and thedifferent output thereof is provided, noise components may be canceledby one another. Thus, the input arrangement is a noise-resistant datatransfer input arrangement.

As a result, also in the second embodiment, the entirety of the pulsewidth generating unit and data modulation unit (pulse width modulationand intensity modulation signal generating unit) 11 and thesemiconductor laser control unit and semiconductor unit driving unit(semiconductor laser control and driving unit) 12 can be formed as onechip of an integrated circuit 20 by bipolar transistors. In driving andcontrolling the semiconductor laser in pulse width and intensitycombined modulation method within one dot using the combination of thenegative feedback loop and the addition current value control system(such as that, advantage of which was described with reference to FIGS.2A and 2B), it is possible to achieve miniaturization of the system andenergy saving. Because all the necessary processing is performed withinthe one chip of the integrated circuit 20, the processing can beperformed at high speed with high accuracy.

A third embodiment of the present invention will now be described withreference to the drawings. In the third embodiment, the above-describedpulse width and intensity combined modulation method and combination ofthe negative feedback loop and the addition current value controlsystem, which combination effectively reduces a load carried by thenegative feedback loop, are used. The same reference numerals as thoseshown in FIGS. 1 and 3 are given to identical components/parts in thethird embodiment.

A semiconductor laser control system 313 in the third embodiment, inoutline, includes, as shown in FIG. 3, the pulse width generating unitand data modulation unit 11 (pulse width modulation and intensitymodulation signal generating unit) and semiconductor laser control unitand semiconductor laser driving unit 12 (semiconductor control anddriving unit). The semiconductor laser control unit and semiconductordriving unit 12, as shown in FIG. 1, mainly includes the negativefeedback loop 3 and current driving unit 4. With reference to FIG. 1,data which already has undergone PWM modulation through the pulse widthgenerating unit and data modulation unit 11 are input to theconstant-current sources 5 and 8. The current I_(DA1) of theconstant-current source 5 forms the negative feedback loop 3 togetherwith the inverting amplifier 6, semiconductor laser 1 and lightreception device 2. The current I_(DA2) of the constant-current source 8becomes a forward current of the semiconductor laser 1, and is convertedinto light output of the semiconductor laser 1 at high speed. Thereby,high-speed driving and control of the semiconductor laser 1 can beperformed. In this case, by setting a value of the current I_(DA2) ofthe constant-current source 8 which functions as the current drivingunit 4, that is, a value of the light output Ps, to be a predeterminedvalue, it is possible to perform high-speed PWM and PM modulation oflight output of the semiconductor laser 1 as described above.

FIG. 21 shows an example of a detailed arrangement of the semiconductorlaser control system 313 in the third embodiment. Also in the thirdembodiment, almost all components/parts of the pulse width modulationand intensity modulation signal generating unit 11 and the semiconductorcontrol and driving unit 12 are formed as one chip of an integratedcircuit 320. In more detail, as examples of partial circuit arrangementswill be shown, almost all of the components/parts are formed as one chipby bipolar transistors. The pulse width generating unit and dataconverting unit 11 of the third embodiment is similar to the pulse widthgenerating unit and data converting unit 11 of the second embodimentshown in FIG. 18. Duplicated descriptions thereof will be omitted. Thepulse width generating unit and data converting unit 11 in the thirdembodiment is also formed by bipolar transistors.

A light emission instruction signal setting unit 321, a light emissionsignal generating unit 322, an error amplifier 323, a current drivingunit 324 (voltage shifting unit 325) of the semiconductor laser controlunit and semiconductor driving unit 12 of the third embodiment aresimilar to the light emission instruction signal setting unit 221, alight emission signal generating unit 222, an error amplifier 223, acurrent driving unit 224 (voltage shifting unit 225) of thesemiconductor laser control unit and semiconductor driving unit 12 inthe second embodiment. Duplicated descriptions thereof will be omitted.

The error amplifier 323, voltage shifting unit 335 and light emissioninstruction signal generating unit 322 may include a bipolar transistorelectronic circuit such as that, for example, shown in FIG. 15 of theerror amplifier 223, voltage shifting unit 235 and light emissioninstruction signal generating unit 222 of the second embodiment.Duplicated description thereof will be omitted.

Further, a timing generating unit 331, a differential quantum efficiencydetecting unit 332, a memory unit 333 and a adding current setting unit334 of the third embodiment are similar to the timing generating unit231, differential quantum efficiency detecting unit 232, memory unit 233and adding current setting unit 234 of the second embodiment. Duplicateddescriptions thereof will be omitted.

A starting-up unit 335 is connected with the timing generating unit 331.The starting-up unit 335 provides protection of the semiconductor laser1 from being degraded or damaged due to an excess current flowingtherethrough during the period to the time a power source voltagereaches a predetermined value. Further, the starting-up unit 335generates an initialization starting signal which is necessary in thetiming generating unit 331. The starting-up unit 335 sets a set voltagewhich is approximately the predetermined voltage of the power sourcevoltage. For example, when the predetermined voltage of the power sourcevoltage is 5.0 volts, the set voltage is set to be 4.5 volts. It isdetermined that all of the electronic circuit performs normal operationswhen the power source voltage reaches 4.5 volts. Then, the semiconductorlaser 1 can be protected and the initialization starting signal can bepositively generated. Specifically, the terminal voltage of the lightreception device 2 is forcibly made to be at the H level, and thereby,the output of the error amplifier 323 is forced to the L level. Thus, aforward current is prevented from flowing through the semiconductorlaser 1, and the semiconductor laser 1 is protected. Similarly, asdescribed later, the voltage of a TDSTART terminal is forcibly set tothe H level, and thereby an oscillator circuit (described later) in thetiming generating unit 331 is prevented from performing oscillation.Then, when the power source voltage Vcc reaches the set voltage or abovethe set voltage, the protection of the semiconductor laser 1 iscanceled, and the control circuit of the semiconductor laser 1 is in anormal operation state. Also, the oscillation prevention of theoscillation circuit in the timing generating circuit 331 is released,and an oscillation starting signal is generated. Similarly, a VPTDSTARTterminal voltage which produces a current source of the timinggenerating circuit 331 is output.

It is possible that the timing generating circuit 331 is formed using adelay circuit. However, in the embodiment, the timing generating circuit331 includes the oscillation circuit 336, a bias circuit (not shown inthe figure) and a latch circuit 337. In outline, an oscillation signalgenerated by the oscillation circuit 336 is latched by the latch circuit337, the latched data is forwarded to a subsequent stage, one by one.Thereby, for example, 6 timing signals T0, T1, . . . , T5 are generated.The oscillation circuit 336 is forcibly prevented from performingoscillation when the last timing signal is generated.

The differential quantum efficiency detecting unit 332 includes, forexample, a sample-and-hold circuit 338 which detects a peak value oferror output of the error amplifier 323, and a comparing circuit 339which compares an output value of the sample-and-hold circuit with apredetermined value.

The memory unit 333 holds comparison results of the comparing circuit339 in synchronization with the timing signals T1, T2, . . . , T5 of thetiming generating circuit 331. The adding current setting unit 334includes, for example, a 5-bit digital to analog converter 340.

Arrangements and functions of these units will now be described. FIG. 22shows an example of circuit arrangement of the oscillation circuit 336by bipolar transistors. FIG. 24 shows outline operation in aninitialization time. A collector voltage V_(Q22C) of a transistor Q₂₂ (avoltage of the terminal TDSTART) is shown as an oscillation operation inFIG. 5. A collector current of the transistor Q₂₂ is made to turn on andturn off by a differential switch including transistors Q₂₄, Q₂₅. In acase where the collector current of the transistor Q₂₂ is larger than acollector current of a transistor Q₂₁ when the collector current of thetransistor Q₂₂ is turning on, the collector voltage V_(Q22C) of thetransistor Q₂₂ decreases as a result of a capacitor C₁ being dischargedby the differential current of the collector currents of the transistorsQ₂₁ and Q₂₂. When the collector current of the transistor Q₂₂ turns off,the capacitor C₁ is charged by the collector current of the transistorQ₂₁ and the collector voltage V_(Q22C) increases. The changing anddischarging of the capacitor C₁ is repeated alternately and oscillationis performed.

A time from a timing 0, that is, from power source supply starting to anoscillation starting timing signal TS is sent from the starting-up unit335, the voltage of the TDSTART terminal is forcibly set at the H level(approximately the same as Vcc), and the voltage of the VPTDSTARTterminal is 0 volts. Thereby, a collector current of a transistor Q₂₃ is0. With reference to FIG. 23B showing a last stage of a latch circuit348, when the voltage of the VPTDSTART is 0, a collector current of atransistor Q₃₁ is 0. As result, a base of the transistor Q₂₃ is Vcc, andthe collector current of the transistor Q₂₃ is 0. In the differentialswitch 346, because a collector current of a transistor Q₂₃ is 0although the base of the transistor Q₂₅ is at the L level, the collectorcurrent of the transistor Q₂₂ is 0. Then, after the oscillation startingsignal TS is sent and the voltage of the VPTDSTART terminal is at the Hlevel, the collector current of the transistor Q₂₂ starts flowing. Inthe differential switch 346, because the base of the transistor Q₂₅ isat the L level, the collector current of the transistor Q₂₃ flowsthrough the transistor Q₂₆. Through a current-mirror circuit 347including transistors Q₂₆ Q₂₂ the same current flows through thetransistor Q₂₂. At the timing TS, when the collector current of thetransistor Q₂₂ is larger than the collector current of the transistorQ₂₁, the collector voltage V_(Q22C) of the transistor Q₂₂, that is, thevoltage of the terminal TDSTART terminal gradually decreases. Then, whenthe base voltage of the transistor Q₂₄ becomes equal to or lower thanthe base voltage of the transistor Q₂₅, the differential switch 346operates instantaneously thus the transistor Q₂₄ turns on, the collectorcurrent of the transistor Q₂₆, and thus, the collector current of thetransistor Q₂₂ turn off. The base voltage of the transistor Q₂₅ rises bya electrical potential which is determined by the collector current ofthe transistor Q₂₄ and a resistor R₁₁. This moment is the timing T0.

After the timing T0, because the collector current of the transistor Q₂₂turns off, the collector voltage V_(Q22C), that is, the voltage of theTDSTART terminal gradually increases. When the base voltage of thetransistor Q₂₄ becomes equal to or higher than the base voltage of thetransistor Q₂₅, the differential switch 346 operates inversely, and thecollector current of the transistor Q₂₂ turns on. Thus, oscillation isperformed. An amplitude of the oscillation is determined by the voltagedetermined by the collector current of the transistor Q₂₄ and theresistor R₁₁. The period of the oscillation is determined by thecollector current of the transistor Q₂₁, the collector current of thetransistor Q₂₂ and the capacity of the capacitor C₁. By determiningthese values, it is possible to obtain desired timing signals.

In this operation, when the collector current of the transistor Q₂₂ isexactly twice the collector current of the transistor Q₂₁, (thecollector current of the transistor Q₂₁)=(the collector current of thetransistor Q₂₂)-(the collector current of the transistor Q₂₁). Thereby,the change amounts per unit time in cases where the capacitor C₁ ischarged and discharged are equal to one another. Therefore, as shown inFIG. 24, V_(Q22C) varies to form a triangle wave such that the risingtime is equal to the decaying time.

A rectangular wave is obtained at the base of the transistor Q₂₅ asoscillation output of the oscillation circuit 336. Then, after therectangular wave undergoes voltage shifting, swing amount adjustment,and inverting, an output waveform, an emitter voltage V_(QXE) of atransistor Q_(X) (not shown in the figure) is obtained. The waveform theV_(QXE) is obtained as a result of converting the triangle wave ofV_(Q22C) into a two-level signal through a known manner.

FIG. 23 shows an example of a circuit arrangement of a latch circuit 348as one constituent unit of the latch circuit 337. The latch circuit 337includes 6 stages of latch circuits 348 which are connected forgenerating the timing signals T0, T1, T2, . . . , T5. The latch circuit348 as one constituent unit thereof generates the timing signal T0. Theexample of circuit arrangement is formed by a plurality of transistorsand resistors. Transistors Q₃₁, Q₃₂, Q₃₃ form one switch 349a, andtransistors Q₃₄, Q₃₅, Q₃₆ form another switch 349b. In the switch 349a,when the collector current of the transistor Q₃₃ turns on, the basevoltage of the transistor Q₃₁, that is, input data is output as the basevoltage and the emitter voltage of a transistor Q₃₇. In the switch 349b,when the collector current of the transistor Q₃₆ turns on, because thebase voltage of the transistor Q₃₄ is connected to the emitter of thetransistor Q₃₇, the output is maintained as it has been.

Assuming an input signal to the base of the transistor Q₃₃ is referredto as CLK, an input signal to the base of the transistor Q₃₆ is referredto as CLK, an input data to the base of the transistor Q₃₁ is referredto as DATA0, and output at the emitter of the transistor Q₃₇ is referredto as Q, the following logic equation is obtained:

    Q=CLK·DATA0+CLK·Q.

As described above, the emitter voltage V_(QXE) of the transistor Q_(X)is maintained as being at the H level between the timing TS and thetiming T0, as shown in FIG. 24, and is the base voltage CLK of thetransistor Q₃₆. Further, the current of a current source 350 includingtransistors Q₃₈, Q₃₉ is 0 until the timing TS, and it flowsinstantaneously from the timing TS. The output Q is at the H level untilthe timing T0 because the signal CLK is at the H level and the output Qis maintained as it has been. At the timing T0, the output Q is at the Llevel for the first time. This is because the signal CLK (V_(QXE)) is atthe L level and the L level is input to the base of Q₃₁ as DATA0. Afterthe timing T0, because the input data of the base of the transistor Q₃₁is maintained to be at the L level, the output Q is maintained as beingat the L level. This function of the emitter voltage V_(Q37E) is shownas V_(Q37E) (timing signal T0) in FIG. 24.

In the subsequent stage (not shown in the figure), the signals CLK andCLK are connected inversely, and the emitter voltage V_(Q37E) is used asthe input data DATA1. Then, the following logic equation is obtained forthe output Q':

    Q'=CLK·DATA1+CLK·Q'.

Thereby, the timing signal T1 shown as V_(Q37)(1)E in FIG. 24 can beobtained. In fact, the switch 349a of the subsequent stage latches andoutputs the input data DATA1 (L level) at the timing of the rising edgeof CLK, and the L level output is maintained because the L level of theinput data of DATA1 is maintained.

Similarly, the timing signal T2, T3, T4 and T5 can be obtained and areshown as V_(Q37)(2)E, V_(Q37)(3)E V_(Q37)(4)E and V_(Q37)(5)E in FIG.24, respectively. `n` in V_(Q37)(n)E in FIG. 24 represents the stagenumber 1, 2, 3, 4, 5.

In the last stage of latch circuit 348 shown in FIG. 23B which generatesthe timing signal T5, the collector current of the transistor 31 isconnected, via a transistor as shown in FIG. 23B, to the base of thetransistor Q₂₃ in the oscillation circuit 336, and is used as a voltagefor driving the oscillation circuit 336. The base voltage of thetransistor Q₂₃ is provided from the timing TS to the timing T5. However,the base voltage of the transistor Q₂₃ is not provided, and thecollector current of the transistor Q₂₃ is turned off at the moment ofthe timing T5.

Thus, oscillation circuit 336 performs oscillation only during a timewhen the necessary timing signals are generated. When generation of thenecessary timing signals is finished, the oscillation is stoppedsimultaneously. Thereby, the oscillation operation of the oscillationcircuit 336 does not adversely affect, through noise, current variationor the like, or other electronic circuits. It is also possible to use adelay circuit or the like for generating the timing signals T0, T1, T2,. . . , T5. However, by using the oscillation circuit 336, by merelyexternally connecting the capacitor C₁ to the LSI circuit (integratedcircuit 320), timing of the oscillating circuit 336 can be freely seteven in the case where many timing signals are generated. If the timinggenerating circuit 331 is formed using a delay circuit, an externallyconnecting component is needed for determining each timing when thetiming is freely set. However, when the number of timing signals to begenerated is small, using a delay circuit may be advantageous becausethe latch circuit is not required. Anyway, the control speed of thenegative feedback loop 3 can be freely set, and also, it is possible toobtain light output without being affected by frequency characteristicsof the semiconductor laser 1 and light reception device 2. Further,providing the timing generating unit 331 is advantageous for determiningthe initialization time of the integrated circuit 320 to be an optimumtime.

Generally speaking, there are frequency characteristics between thesemiconductor laser 1 and the light reception device 2. In a case wherethe frequency characteristics adversely affect operation of the negativefeedback loop 3 and/or the above-described timing setting operation, itis necessary to add an electronic circuit for compensating the frequencycharacteristics. If such a compensating circuit is not added, it isnecessary to set the above-described timing to be sufficiently delayed.However, by making the timing delayed, the initialization time of theintegrated circuit 320 is elongated. If such a compensating circuit isadded, the number of components of the integrated circuit 320 increases.By providing the timing generating unit 331 using the oscillationcircuit 336, without needing such a compensating circuit, by merelychanging the capacitance of the capacitor C₁, the timing can beeffectively adjusted. Thus, efficient initialization can be performedwithout increasing the number of components. Further, in comparison towhere flip-flops are used for generating such timing signals, by usingthe latch circuit 337 of the combination of the necessary stages oflatch circuit 348 in the embodiment, it is possible to reduce the numberof components.

An outline of operations controlled by these timing signals in theinitialization time will be described with reference to the time chartof FIG. 5 and FIG. 6 showing an example of a circuit arrangement of thedifferential quantum efficiency detecting unit 332. At the timing TS,light output of the semiconductor laser 1 is forcibly changed to apredetermined maximum light emission condition from an off condition.This maximum light emission value was already set in the light emissioninstruction signal generating unit 322. At the timing T0, all the inputdata is made to be 0, and thus, an offset light emission condition isset. This condition is maintained until the timing T5. After that, anormal operation condition is set where ordinary input data is accepted.In order to maintain an operation of the negative feedback loop 3, lightoutput of the semiconductor laser 1 is not completely off, and it isnecessary to set the offset light emission condition where a slightlight output is provided. Actually, light output of the semiconductorlaser 1 is controlled by the negative feedback loop 3 between themaximum light emission condition and the offset light emissioncondition.

In the initialization time, that is, in power source supply startingtime or a reset time, a sequence operation such as that shown in FIG. 24is performed. Thus, the differential quantum efficiency is detected eachtime, and suitable adding current value is set for light output of thesemiconductor laser 1.

A ratio of the difference between the maximum light emission lightoutput power and the offset light emission light output power shown inFIG. 24 to the difference, which is (operation current Iop)-(LDoscillation threshold current Ith), is the differential quantumefficiency. The sample-and-hold circuit 338 of the differential quantumefficiency detecting unit 332 detects the difference in the current. Inoutline, this difference corresponds to a difference of a voltagebetween the two terminals of the resistor Re shown in FIG. 21 betweenthe cases of maximum light emission and offset light emission. When thevoltage shifting unit 325 of the current driving unit 324 does notoperate, the difference corresponds to a difference of an emittervoltage of a transistor Q₁₂ (shown in FIG. 15) of the error amplifier324 (corresponding to the error amplifier 224 in the second embodiment)between the two cases. Therefore, the emitter voltage of the transistorQ₁₂ is sampled and held at the time of the maximum light emission. Then,the amount of voltage shift of the voltage shifting unit which is 0 atthe timing T0 is gradually changed through the adding current settingunit 334, and the differential quantum efficiency is detected as avoltage difference in the resistor R₂ (shown in FIG. 15) in the voltageshifting unit.

In detail, the emitter voltage of the transistor Q₁₂, that is, thevoltage of a VCOMP terminal becomes the base voltage of a transistor Q₄₃via an emitter follower 251 of a transistor Q₄₂ shown in FIG. 25. Whilea current of a current source 352 including a transistor Q₄₅ is flowing,the base voltage of the transistor Q₄₃ is equal to the base voltage of atransistor Q₄₄ via a voltage follower 353 including transistors Q₄₁,Q₄₆, Q₄₇, Q₄₈. At the timing T0, the current of the current source 352is made to turn off. After that, a change in the base voltage of thetransistor Q₄₃ indicates a change in the voltage of the VCOMP terminal.However, by using a large capacity of a capacitance C₂, the base voltageof the transistor Q₄₄ hardly changes, and it is possible to sample andhold the base voltage of the transistor Q₄₃ at the timing T0, that is,the emitter voltage of the transistor Q₁₂ at the time of maximum lightemission. FIG. 24 shows outlines of waveforms of voltage variations ofthe base voltages of the transistors Q₄₃, Q₄₄.

These base voltages of the transistors Q₄₃, Q₄₄ are input to andcompared by a comparator 339 including transistors Q₄₉, Q₅₀. Results ofthe comparisons are stored in the memory unit 333 in synchronizationwith the timing signals T1, T2, T3, T4 and T5. Not shown in the figures,the memory unit 333 has functions of storing the comparison results insynchronization with the timing signals T1, T2, T3, T4 and T5. Forexample, the memory unit 333 may include five stages of latch circuitssuch as those used in the timing generating unit 331. In this case, whenthe base voltage of the transistor Q₄₃ is higher than the base voltageof the transistor Q₄₄ in the comparison, the L level is output to alatch circuit.

The adding current setting unit 334 includes five sets of switches, eachset including two stages of differential switches, current mirrorcircuits which supply currents to current sources of these switches, anda current mirror circuit which sums the respective sets of switches andoutput the summed result to the current driving unit (voltage shiftingunit 325). The five sets of switches basically form a 5-bit digital toanalog converter 340. The current source of each set of switches is setas follows: Assuming that the current for the least significant bit isI1, the currents for the subsequent bits are 2·I1, 4·I1, 8·I1, 16·I1,respectively, in the order of less significance. Thereby, the totaloutput current of the all the switch sets is 31·I1 which is a maximum.The corresponding maximum current (voltage) of the current driving unit(voltage shifting unit 325) is set so that the maximum current is largerthan the maximum value of the above-mentioned (operation currentIop)-(LD oscillation threshold current Ith).

At the timing T0, light output of the semiconductor laser 1 is changedfrom the maximum light emission condition to the offset light emissioncondition, and simultaneously, the current of the most significant bitof the above-mentioned switch set is forcibly output. Thereby, a voltagechange occurs in the voltage shifting unit. Then the control system ofthe negative feedback loop 3 operates so that the light output should bethe offset light emission condition although the voltage changes in thevoltage shifting system. In other words, the negative feedback loop 3operates so that such a voltage change in the voltage shifting unit dueto the output of the current of the most significant bit of the switchset is canceled and the offset light emission condition results. Thus,the voltage of the VCOMP terminal changes. Such a change is detected bythe differential quantum efficiency detecting unit 332, and the VCOMPterminal voltage at this time is compared with the VCOMP terminalvoltage at the time of the maximum light emission. A result of thecomparison is stored in the memory unit 233. The memory unit 233 latchesthe comparison result and sets the set of the switches for the mostsignificant bit of the adding current setting unit 234. In the setting,when the VCOMP terminal voltage at the time of forcibly outputting thecurrent is larger than that at the time of the maximum light emission,the setting is OFF. When the VCOMP terminal voltage at the time offorcibly outputting the current is smaller than that at the time of themaximum light emission, the setting is ON. The time T0 to T1 (also thetime T1 to T2, . . . , the time T4 to T5) should be set so thatoperation of the control system of the negative feedback loop 3 becomescompletely stable and thus a change in the output value completelyconverges.

At the timing T1, similar to the case at the timing T0, the current ofthe second significant bit of the set of switches of the adding currentsetting unit 334 is forcibly output. Then, at the time T2, comparison ofthe VCOMP terminal voltage at the time with that of the maximum lightemission is performed, and the switch setting for the bit is set to beON or OFF according to a result of the comparison. In this embodiment,the differential quantum efficiency detection is performed with anaccuracy of 5-bit digital to analog conversion. Therefore, a similaroperation is repeated five times. During these operations, the basevoltage of the transistor Q₄₃ varies as shown in FIG. 24. Thus, the 5bits of the set of switches of the addition current setting unit 334 areappropriately set. In the example of the figure, the setting result isas follows in the order of the bit significance:

1, 1, 1, 0, 1.

In the embodiment, detection accuracy of the differential quantumefficiency detection unit 332 and the addition current setting unit 334is 5 bits. However, it is possible to increase the number of bits so asto improve the detection accuracy. Thereby, in the light output waveformshown in FIG. 2B, it is possible that the light output of Ps furtherapproximates a desired light output. Thereby, the control operation ofthe control system of the negative feedback loop 3 is reduced, and thelight output further approximates an ideal rectangular wave.

Further, in the embodiment, the current driving unit 324 is formed bythe voltage shifting unit 325 which is inserted in the negative feedbackloop 3. However, it is also possible that, as shown in FIG. 26, thecurrent driving unit 324 is separate from and independent of thenegative feedback loop 3. Further, with regard to the light emissioninstruction signal generating unit and light emission instruction signalsetting unit, as shown in FIG. 27, it is possible to separate them intothe light emission instruction signal generating unit 322a and lightemission instruction signal setting unit 321a for I_(DA1) and the lightemission instruction signal generating unit 322b and light emissioninstruction signal setting unit 321b for I_(DA2). In this case, thelight emission instruction signal setting unit 321b is the additioncurrent setting unit 334.

Further, in the embodiment, almost all of the semiconductor lasercontrol system 313 is formed as an integrated circuit 320 using bipolartransistors. However, it is also possible that the system is formed asan integrated circuit using C-MOS transistors. Further, it is alsopossible that the system is formed as an integrated circuit using acombination of bipolar transistors and C-MOS transistors. Furthermore,it is also possible that the system is formed as a non-integratedcircuit.

A fourth embodiment of the present invention will now be described withreference to the drawings. The fourth embodiment is similar to the thirdembodiment described above. Duplicated descriptions will be omitted.

FIG. 28 shows an example of a detailed arrangement of the semiconductorlaser control system 413 in the fourth embodiment. The pulse widthgenerating unit and data modulation unit 11 generate the light emissioninstruction signal which simultaneously performs pulse width modulationand intensity modulation according to image data (input data). The lightemission instruction signal is a plurality of pulses which include pulsewidth modulation data and intensity modulation data.

With regard to the semiconductor laser control and driving unit 12, thenegative feedback loop 3 generally includes a light emission instructionsetting unit 421, a light emission instruction signal generating unit422, an error amplifier 423, a current driving unit 424, thesemiconductor laser 1 and the light reception device 2. This arrangementand operation is similar to that of the negative feedback loop 3 of thethird embodiment including the light emission instruction setting unit321, light emission instruction signal generating unit 322, an erroramplifier 323, a current driving unit 324, the semiconductor laser 1 andthe light reception device 2. However, in the fourth embodiment, asshown in FIG. 28, similar to FIG. 26, the current driving unit 424 isactually separate from and independent of the negative feedback loop 3.

Further, a timing generating unit 431, a differential quantum efficiencydetecting unit 432 (including a sample-and-hold circuit 438 and acomparator 439), a memory unit 433, a adding current setting unit 434(of a 5-bit digital to analog converter 440, for example) and astarting-up unit 435 of the fourth embodiment are similar to the timinggenerating unit 331, differential quantum efficiency detecting unit 332(including the sample-and-hold circuit 338 and comparator 339), memoryunit 333, adding current setting unit 334 (of the 5-bit digital toanalog converter 340, for example) and starting-up unit 335 of the thirdembodiment. Duplicated descriptions thereof will be omitted.

Further, a oscillation circuit 436 and a latch circuit 437 of the fourthembodiment is similar to the oscillation circuit 336 and latch circuit337 of the third embodiment. Duplicated description thereof will beomitted.

The pulse width generating unit and data modulation unit 11 will be nowdescribed. In the embodiment, the pulse width modulation uses 3 bits(that is, 8 levels) and the intensity modulation uses 5 bits (that is,32 levels). By combining the two modulations, 8-bit tone (256 levels)can be output. In outline, the pulse width generating unit and datamodulation unit 11 includes a pulse width modulation and intensitymodulation signal generating unit 461 and a light emission instructionsignal generating unit 422. In outline, the pulse width modulation andintensity modulation signal generating unit 461 includes a pulsegenerating unit 462 which includes a PLL and generates a plurality ofpulses of different timing, a data converting unit 463 which includeslogic units and converts input image data into pulse width modulationdata and intensity modulation data, and a pulse width modulation unit464 which selects pulses from pulses output from the pulse generatingunit 462 according to the pulse width modulation data obtained from thedata converting unit 463. The pulse width modulation and intensitymodulation signal generating unit 461 performs the above-described logicoperation and is formed of electronic circuits by bipolar transistors.

The light emission instruction signal generating unit 422 includes, asshown in FIG. 29A, a digital to analog converter (DAC) 465 whichgenerates the currents I_(DA) and I_(DA) according to intensitymodulation data PMDATA (PMD), a differential switch 466 which allows ordoes not allow the current I_(DA) to flow therethrough according to apulse 1, a differential switch 467 which allows or does not allow thecurrent I_(DA) to flow therethrough according to a pulse 2. It is notedthat I_(DA) +I_(DA) =I_(full). The value of I_(full) is the value ofI_(DA) when all of PMDATA is ON, and is the maximum current value of thelight emission instruction signal. The differential switches 466, 467function so that, when each of the pulse 1 and pulse 2 is at the Hlevel, I_(DA1) =I_(full). When the pulse 1 is at the L level and thepulse 2 is at the H level, I_(DA1) =I_(DA). When each of the pulse 1 andpulse 2 is at the L level, I_(DA1) =0. When each of the pulse 1 andpulse 2 is at the H level, I_(DA1) =I_(full), without depending on thevalue of I_(DA) (that is, without depending on PMDATA). Therefore, theintensity modulation data PMDATA can be fixed during one pixel clockperiod. This is advantageous in achieving a high-speed operation of thesemiconductor laser control system. Each of such differential switchesas the switches 466, 467 can be formed as a result of differentialconnection of a pair of bipolar transistors. Therefore, the lightemission instruction signal generating unit 422 itself can be easilyformed as an integrated circuit by bipolar transistors.

A case will now be considered where 8-bit tone (256 levels) is outputfor one dot and 5 bits (32 levels) are used for the intensitymodulation. The maximum current supplied by the 5-bit digital to analogconverter 465 is 31·I₀ (where I₀ is a current flowing for the leastsignificant bit). The current 31·I₀ is set as the maximum currentI_(full). In the arrangement shown in FIG. 29A, data 31/256 and data32/256 results in a same output. Similarly, data 63/256 and data 64/256results in a same output, . . . , data 223/256 and data 224/256 resultsin a same output. As a result, the number of tone levels for one dot is249. In order to solve this problem, as shown in FIG. 29B, aconstant-current source 468 which always supplies the current I₀ to thedifferential switch 466 is added and setting such that I_(full) =32·I₀is performed. Thereby, it is possible that 256 tone levels, 0/256 to255/256 are obtained, and the number of tone levels increases. However,when all image data is at the H level, the semiconductor laser 1 doesnot fully turn on (256/256). (In the case of FIG. 29A, the semiconductorlaser 1 fully turns on because data 255/256 and data 256/256 result inthe same output.) In order to solve this problem, as shown in FIG. 29C,a differential switch 469 is provided. This switch 469 allows a currentto flow to the switch 466 only when a full on signal which is at the Hlevel only when all image data is at the H level. The switch 469 allowsa current to flow to the switch 467 at all the other times. Thereby,although the number of devices increases, 256 tone levels, 0/256 to254/256, 256/256 can be obtained. Thus, depending on a particularpurpose, one of the three arrangements shown in FIGS. 29A, 29B and 29Cmay be selected.

As mentioned above, as shown in FIG. 28, the pulse width modulation andintensity modulation signal generating unit 461 includes the dataconverting unit 463 which is data converting means, the pulse widthmodulation unit 464 which is pulse width modulating means and the pulsegenerating unit 462 which includes a PLL and is pulse generating means.The pulse generating unit 462 has a function the same as the function ofthe above-described pulse generating oscillator 118 in the firstembodiment, and supplies pulses X₀, X₁, X₂, . . . , X₇ shown in FIG. 13.Duplicated description thereof will be omitted. The pulse generatingunit 462 includes a phase frequency comparator, a voltage controlledoscillator and a lowpass filter.

The data converting unit 463 and the pulse width modulation unit 464 mayhave functions the same as those of the data converting unit 116 andpulse width modulation unit 117 of the first embodiment shown in FIG.12A. The data converting unit converts input image data into pulse widthmodulation data PWMDATA and intensity modulation data PMDATA, and thepulse width modulation unit 464 generates two pulse signals PW_(on)(pulse 2) and PW_(da) (pulse 1) from X₀, X₁, X₂, . . . , X₇ output fromthe pulse generating unit 462 according to the pulse width modulationdata PWMDATA obtained from the data converting unit 463.

In the fourth embodiment, as shown in FIG. 28, a switch unit 470 isinserted between the light emission instruction signal generating unit422 and the error amplifier 423, and current driving unit 424. Thisswitch unit 470 is formed by bipolar transistors. The light emissioninstruction signal provided to the error amplifier 423 and currentdriving unit 424 from the light emission instruction signal generatingunit 422 is changed. Thereby, it is possible that the semiconductorlaser 1 is forcibly caused to emit light or forcibly caused to ceaseemitting light without regard to image data.

Specifically, in a case where the arrangement shown in FIG. 29A isselected as the light emission instruction signal generating unit 422,an arrangement shown in FIG. 30 is provided as the light emissioninstruction signal generating unit 422 and the switch unit 70. In thearrangement of FIG. 30, a switch 470a allows or does not allow a currentto flow therethrough according to a forcible light emission instructionsignal. A switch 470b allows or does not allow a current to flowtherethrough according to a forcible light cessation instruction signal.The two switches 470a and 470b are connected to the switches 466 and 467and change the light emission instruction signal to be provided to theerror amplifier 423 and current driving unit 424. The switch 466 allowsa current to flow therethrough either only to the switch 470a or only tothe switch 470b, according to the pulse 2. The switch 467 allows acurrent to flow therethrough either only to the switch 470a or only tothe switch 470b, according to the pulse 1. The forcible light emissioninstruction signal and forcible light cessation instruction signal areappropriately generated and output by the timing generating unit 431. Inthe embodiment, when the semiconductor laser 1 differential quantumefficiency detection operation is performed as mentioned above, theforcible light emission instruction signal and forcible light cessationinstruction signal are generated. Thereby, without regard to image data(pulse 1, pulse 2), the semiconductor laser 1 is forcibly caused to emitlight or forcibly caused to cease emitting light. However, the forciblelight emission instruction signal and forcible light cessationinstruction signal are not at the H level simultaneously. Further, eachof the switches 466, 467, 470a and 470b is formed of a ECL (emittercoupled logic) circuit, where the emitters of a pair of transistors areconnected to one another, by bipolar transistors.

In the arrangement shown in FIG. 30, in the above-described differentialquantum efficiency detection operation, the forcible light cessationinstruction signal is caused to be at the H level, and also, theforcible light emission instruction signal is caused to be at the Llevel. Thereby, the switches 470a and 470b allow neither a current I nora current I, supplied via the switches 466 and 477, to flowtherethrough. As a result, without regard to image data, thesemiconductor laser 1 is forcibly caused to be the off condition. Then,the forcible light cessation instruction signal is caused to be at the Llevel, and also, the forcible light emission instruction signal iscaused to be at the H level. The switches 470a and 470b allow both thecurrent I and the current I, supplied via the switches 466 and 477, toflow therethrough. As a result, without regard to image data, thesemiconductor laser 1 is forcibly caused to be at the maximum lightemission condition. Thus, the condition of the semiconductor laser 1 ischanged. Thereby, the semiconductor laser 1 differential quantumefficiency detection operation, described above with reference to FIG.25, can be performed independently, arbitrarily, without regard to imagedata. In a normal condition, the forcible light cessation instructionsignal is caused to be at the L level, and also, the forcible lightemission instruction signal is caused to be at the L level. Thereby, theswitches only 470b allows only the current I, supplied via the switches466 and 477, to flow therethrough.

In the fourth embodiment, the switch unit 470 is provided between thelight emission instruction signal generating unit 422 and the erroramplifier 423. However, it is also possible that the switch unit 470 isprovided between the light emission instruction signal generating unit422 and pulse width modulation unit 464.

Further, in the embodiment, almost all of the semiconductor lasercontrol system 413 is formed as an integrated circuit 420 using bipolartransistors. However, it is also possible that the system is formed asan integrated circuit using C-MOS transistors. It is also possible thatthe system is formed as an integrated circuit using a combination ofbipolar transistors and C-MOS transistors. Furthermore, it is alsopossible that the system is formed as a non-integrated circuit.

A fifth embodiment of the present invention will now be described withreference to the drawings. The fifth embodiment is similar to the secondembodiment described above. Duplicated descriptions will be omitted.

A semiconductor laser control system in the fifth embodiment, inoutline, also includes, as shown in FIG. 3, the pulse width generatingunit and data modulation unit 11 (pulse width modulation and intensitymodulation signal generating unit) and semiconductor laser control unitand semiconductor laser driving unit 12 (semiconductor control anddriving unit).

An example of the detailed arrangement of the semiconductor lasercontrol system in the fifth embodiment is shown in FIG. 14 which alsoshows the detailed arrangement of the second embodiment described above.The pulse width generating unit and data modulation unit 11 includes apulse generating unit such as the pulse generating unit 118 shown inFIG. 18, a data converting unit such as the data converting unit 116shown in the figure and a pulse width modulation unit such as the pulsewidth modulation unit 117 shown in the figure.

With regard to the semiconductor laser control unit and semiconductordriving unit 12, similar to the arrangement of the second embodiment, asshown in FIG. 14, a negative feedback loop 3 includes a light emissioninstruction signal setting unit 221 and a light emission instructionsignal generating unit 222, which form a pulse width modulation andintensity modulation signal generating unit, an error amplifier 223, acurrent driving unit 224 (high-speed voltage shifting unit 225), thesemiconductor laser 1 and light reception device 2. Theseunits/components in the fifth embodiment function in a manner similar tothe corresponding units/components in the second embodiment functions.

An example of a circuit arrangement of the error amplifier 223 andhigh-speed voltage shifting unit 225 in the fifth embodiment is shown inFIG. 15 which also shows the example of the circuit arrangement of theerror amplifier 223 and high-speed voltage shifting unit 225 in thesecond embodiment.

Further, a timing generating unit 231, a differential quantum detectingunit 232, a memory unit 233 and an addition current setting unit 234 inthe fifth embodiment are similar to the corresponding units in thesecond embodiment, and function in a manner similar to the manner inwhich the units in the second embodiment function.

Further, in an integrated circuit 220 of the fifth embodiment, astarting-up unit 235 is connected to the timing generating unit 231, anda power source unit 561 (not shown in FIG. 14) is provided.

FIG. 31 shows an example of a circuit arrangement of the power sourceunit 561 using bipolar transistors. In the power source unit 561, a bandgap reference is formed in a circuit including transistors Q₅₁, Q₅₂,resistors R₂₁, R₂₂, R₂₃, and emitter areas of transistors and resistancevalues are determined so that

    V=(an emitter voltage of Q.sub.53)-V.sub.be,

(where V_(be) : a base-emitter voltage of a transistor) varies as littleas possible due to temperature change. As a result, the emitter voltageof each of transistors Q₅₄, Q₅₅ and Q₅₆ is a stable voltage which doesnot have temperature characteristics. In the circuit shown in FIG. 31, acurrent the same as the current flowing through the transistor Q₅₄ and aresistor R₂₄ is obtained through a current mirror circuit 563. A currentsource can be produced, which is used in the integrated circuit 220. Inthe integrate circuit 220, currents which flow through PNP transistors,in the starting-up circuit 235 which will be described later, which havebase voltages supplied by a VBBP terminal become currents ofconstant-current sources, respectively. Similarly, currents which flowthrough NPN transistors which have base voltages supplied by a VBBNterminal become currents of constant-current sources, respectively.Specific current values are determined by resistances which areconnected to the emitters of the transistors, respectively.

The starting-up unit 235 will now be described. The starting-up unit 235protects the semiconductor laser 1 from being degraded or damaged due toan excessive current flowing therethrough during a time in which a powersource voltage Vcc reaches a predetermined value after the power supplyis started. The starting-up unit 235 generates an initializationstarting signal which is necessary in the timing generating unit 231. Asshown in FIG. 32, the starting-up unit 235 includes a first starting-upunit 235a and a second starting-up unit 235b. The second starting-upunit will be described later with a description of the light emissioninstruction signal setting unit 221. In the first starting-up unit 235a,resistances R₃₁ to R₃₇ and so forth are set so that, in a differentialswitch 565 including transistors Q₆₁ and Q₆₂, the transistor Q₆₂ is inits turning-on state from the time the power source voltage Vcc is 0 tothe time Vcc reaches a certain set voltage, and the transistor Q₆₁ is inits turning-on state from the time Vcc exceeds the set voltage to thetime Vcc becomes a predetermined voltage. The set voltage is set to be avoltage as near as possible to the predetermined voltage of the powersource voltage Vcc. For example, a case will now be considered where thepredetermined voltage of the power source voltage is 5.0 volts. In thecase, when a power source voltage reaches 2 to 3 volts, the entirecircuit is not considered to perform a predetermined operation. However,when a power source voltage reaches 4.5 volts, it is considered thatalmost the entire circuit performs the predetermined operation.Therefore, the above-mentioned set voltage is determined to be 4.5volts, for example. After the power source voltage reaches a voltagevery near to the predetermined voltage of the power source voltage, itis possible to effectively protect the semiconductor laser 1 andgenerate the initialization starting signal in a suitable condition.

In detail, with reference to FIG. 32, the base voltage of a transistorQ₆₂ is obtained as a result of performing voltage shifting on thecollector voltage of a transistor Q₆₃ via an emitter follower 566. Thus,the base voltage of the transistor Q₆₂ is determined by the collectorvoltage of the transistor Q₆₃. Similarly, the base voltage of atransistor Q₆₁ is determined by the collector voltage of a transistorQ₆₅ as long as a transistor Q₆₄ is in its turning-off state. Thecollector voltage of the transistor Q₆₃ is determined by a current of acurrent source including a transistor Q₆₆ and a resistor R₃₃ and thepower source voltage. The current of the current source of thetransistor Q₆₆ and resistor R₃₃ is referred to as I₁, the power sourcevoltage is referred to as V_(Vcc), and the collector voltage of thetransistor Q₆₃ is referred to as V_(q63c). Then,

    V.sub.q63c =V.sub.Vcc -I.sub.1 ·R.sub.31.

The current I₁ is a current of a constant-current source having the basevoltage supplied via the terminal VBBN. Therefore, I₁ ·R₃₁ is a constantcurrent. The power source unit 561 is driven by a power source voltage.Therefore, when the power source voltage is 0 volts, the current I₁ is 0amperes. However, because the above-mentioned set voltage is determinedas near as possible to the predetermined voltage of the power sourcevoltage, the power source unit 561 functions adequately and the currentI₁ is a constant current, in a condition (time) where a differentialswitch 565 including the transistors Q₆₁, Q₆₂ performs a switchingoperation. Therefore, V_(q63c) varies according to variation of thepower source voltage V_(Vcc).

The collector voltage V_(q65c) of the transistor Q₆₅ is, similar to theabove equation, expressed by:

    V.sub.q65c =V.sub.Vcc -I.sub.2 ·R.sub.32,

where the current of a current source including a transistor Q₆₇ and aresistor R₃₄ is referred to as I₂. Assuming that resistors R₃₄, R₃₅ haveresistance values the same as one another, when considering a currentflowing through a resistor R₃₆,

    V.sub.Vcc =(I.sub.2 +I.sub.3)·R.sub.36 +V.sub.be +I.sub.2 ·R.sub.35,

where the current I₃ is the current of a constant-current sourceincluding a transistor Q₆₈ and a resistor R₃₇, and V_(be) is thebase-emitter voltage of a transistor.

From the equations,

    V.sub.q65c =I.sub.3 ·R.sub.36 +V.sub.be +I.sub.2 ·(R.sub.36 +R.sub.35 -R.sub.32).

I₃ is a constant current similar to I₁ and thus I₃ ·R₃₆ is a constantvoltage. Further V_(be) is approximately a constant voltage. Therefore,when

    R.sub.36 +R.sub.35 =R.sub.32,

the collector voltage V_(q65c) of the transistor Q₆₅ is a constantvoltage, independent of the power source voltage. Thus, the collectorvoltage V_(q65c) of the transistor Q₆₅ is a constant voltage, and thecollector voltage V_(q63c) of the transistor Q₆₃ varies according tovariation of the power source voltage V_(Vcc). Therefore, byappropriately setting the two voltages, it is possible that thedifferential switch 565 including the transistors Q₆₁, Q₆₂ performs aswitching operation at an appropriate timing according to variation ofthe power source voltage after the power supply is started. As a result,from the timing the power source voltage Vcc is 0 volts to the timingthe Vcc reaches the set voltage, the transistor Q₆₂ is in its turning-onstate. In this condition, a current the same as the collector current ofthe transistor Q₆₂ occurs through a current mirror circuit 567, andthereby, transistors Q₆₉, Q₇₀ turn on. Thereby, each of the voltage of aTDSTART terminal and the voltage of a PD terminal is forced to a voltagethe same as the voltage of Vcc. Specifically, a control operation isperformed such that the PD terminal of the light reception device 2 isforced to be at the H level, and the output of the error amplifier 223is forced to be at the L level. Thereby, no forward current flowsthrough the semiconductor laser 1. Thus, the semiconductor laser 1 isprotected. As a result of the voltage of the TDSTART terminal being atthe H level simultaneously, as described later, an oscillation circuitin the timing generating unit 231 is controlled so that the oscillationcircuit does not oscillate. Then, when the power source voltage Vccexceeds the set voltage, and the transistor Q₆₁ turns on, theabove-mentioned semiconductor laser 1 protection is canceled, and anormal operational state occurs. Further, the control of the oscillationcircuit of the timing generating unit 231 to prevent the oscillationcircuit from oscillating is released. Thus, an oscillation startingsignal is generated. Simultaneously, a VPTDSTART terminal voltage whichcreates a current source of the timing generating unit 231 is output.

The timing generating unit 231 in the fifth embodiment is similar to thetiming generating unit 331 in the third embodiment. The differentialquantum efficiency detecting unit 232 in the fifth embodiment is similarto the differential quantum efficiency detecting unit 332 in the thirdembodiment. The memory unit 233 in the fifth embodiment is similar tothe memory unit 333 in the third embodiment. The adding current settingunit 234 in the fifth embodiment is similar to the adding currentsetting unit 334 in the third embodiment. A duplicate descriptions willbe omitted. Each of these timing generating unit 231, differentialquantum efficiency detecting unit 232, memory unit 233 and addingcurrent setting unit 234 is formed as an integrated circuit by bipolartransistors.

FIG. 22 shows an example of the circuit arrangement of the oscillationcircuit of the timing generating unit 231 in the fifth embodiment, whichalso shows the oscillation circuit 336 in the third embodiment. Power issupplied to the oscillation circuit from the power source unit 561 viathe starting-up unit 235. FIG. 24 shows an outline operation ininitializing in the fifth embodiment, which also shows the outlineoperation in initializing in the third embodiment. FIGS. 23A, 23B showexamples of circuit arrangements of the first stage and the last stageof latch circuits of the timing generating unit 231 in the fifthembodiment, respectively, which also show the corresponding circuits inthe third embodiment. FIG. 25 shows an example of a circuit arrangementof the differential quantum efficiency detecting unit 232 in the fifthembodiment, which also shows the example of the circuit arrangement ofthe differential quantum efficiency unit 332 in the third embodiment.The adding current setting unit 234 in the fifth embodiment is similarto the adding current setting circuit 334 in the third embodiment. Thus,the configuration and functions of the timing generating circuit 231 inthe fifth embodiment are similar to those of the third embodiment.Duplicated description will be omitted.

Examples of circuit arrangements of the light emission instructionsignal setting unit 221 and light emission instruction signal generatingunit 222, which form a pulse width modulation and intensity modulationsignal generating unit, using bipolar transistors in the fifthembodiment are similar to those in the second embodiment described withreference to FIGS. 16 and 17. Duplicated descriptions thereof will beomitted. A stable voltage of a VREF11 terminal generated by the powersource unit 561 is supplied to the light emission instruction signalsetting unit 221.

The base current compensation of the current of the light emissioninstruction signal generating unit 222, described in the description ofthe second embodiment, will now be specifically described, with respectto the least significant bit. The electronic circuit shown in FIG. 17corresponds to the block arrangement shown in FIG. 11 of the firstembodiment. The emitter current of a transistor Q₇₁ (shown in FIG. 16)is referred to as I_(ref), the base current of a transistor Q₇₇ (shownin the figure) is referred to as I_(q77b), the base current of the leastsignificant bit transistor of DAC 113 (shown FIG. 11) corresponding to atransistor Q₈₅ (shown in FIG. 17) is referred to as I_(DACb5), the basecurrent, corresponding to the least significant bit current, of thetransistor of switch 114B (shown in FIG. 11 but not shown in FIG. 16) isreferred to as I_(SWb5), the base current, corresponding to the leastsignificant bit current, of the transistor of the IVC 115B (shown inFIG. 11) corresponding to a transistor Q₈₁ (shown in FIG. 17) isreferred to as I_(IVCb5). The least significant bit current flowsthrough this path. In this condition, if a current mirror circuit 245ideally provides an identical current, a current I_(DA1-5) of the lightemission instruction signal generating unit 22 corresponding to theleast significant bit current is expressed as follows:

    I.sub.DA1-5 =(I.sub.ref +I.sub.q77b)/4-I.sub.DACb5 -I.sub.SWb5 -I.sub.IVCb5,

where the collector current I_(c) flowing through the above-mentionedtransistors of DAC 113, switch 114B and IVC 115B can be approximated asfollows:

    I.sub.c =(I.sub.ref +I.sub.q77b)/4.

Further, the relationship between a base current I_(b) and a collectorcurrent I_(c) is expressed as follows:

    I.sub.c =h.sub.fe ·I.sub.b,

where the current amplification factor is referred to as h_(fe).Therefore, when approximation is performed so that I_(DACb5) =I_(SWb5)=I_(IVCb5),

    I.sub.DA1-5 =(I.sub.ref +I.sub.q77b)/4-(3/h.sub.fe)(I.sub.ref +I.sub.q77b)/4.

Further,

    (3/h.sub.fe)(I.sub.q77b /4)=(3/h.sub.fe)(1/4)(I.sub.c /h.sub.fe)

is sufficiently small in comparison to I_(c),

    I.sub.DA1-5 =(I.sub.ref +I.sub.q77b)/4-(3/h.sub.fe)(I.sub.ref /4).

In order that:

    I.sub.DA1-5 =I.sub.ref /4,

it should be that:

    I.sub.q77b /4=(3/h.sub.fe)(I.sub.ref /4).

Therefore, by causing the emitter current of the transistor Q₇₇ to be asmuch as three times the emitter current I_(ref) of the transistor Q₇₁,

    I.sub.DA1-5 ≈I.sub.ref /4.

Thus, the base current compensation for the transistors through whichI_(DA1-5) flows can be performed.

Further, base current compensation for all the bits are performed. Forexample, the most significant bit will now be considered. Similar to thecase of considering the least significant bit, the base current of themost significant bit transistor of DAC 113 corresponding to a transistorQ₈₆ (shown in FIG. 17) is referred to as I_(DACb1), the base current,corresponding to the most significant bit current, of the transistor ofswitch 114B is referred to as I_(SWb1), the base current, correspondingto the most significant bit current, of the transistor of the IVC 115Bcorresponding to a transistor Q₈₁ is referred to as I_(IVCb1), a currentI_(DA1-1) of the light emission instruction signal generating unit 22corresponding to the most significant bit current is expressed asfollows:

    I.sub.DA1-1 =(I.sub.ref +I.sub.q77b)·4-I.sub.DACb1 -I.sub.SWb1 -I.sub.IVCb1.

By approximation, I_(DACb1) =I_(SWb1) =I_(IVCb1),

    I.sub.q77b ·4=(3/h.sub.fe)·I.sub.ref ·4.

Therefore, by causing the emitter current of the transistor Q₇₇ to be asmuch as three times the emitter current I_(ref) of the transistor Q₇₁,

    I.sub.DA1-1 ≈I.sub.ref ·4.

Thus, the base current compensation for the transistors through whichI_(DA1-1) flows can be performed. By providing the additional currentfor the number of transistors through which the reference current flows,it is possible to prevent error currents from flowing due to the basecurrents of the transistors, and prevent a change in characteristics ofthe transistors from occurring due to the base currents. Thus, basecurrent compensation can be performed easily.

The adjusting, with an external signal, of the reference current of thelight emission instruction signal generating unit 222 and the referencecurrent of the adding current setting unit 234 together was describedabove in the descriptions of the second embodiment.

In addition, for example, when resistors R₄₄, R₄₅ (shown in FIG. 16)have the same resistance values, a voltage V_(VREF) of the terminalVREF11 is equivalent to a voltage V_(VCONT) of the terminal VCONT.Therefore, the emitter voltage V_(q71e) of the transistor Q₇₉ isexpressed as follows:

    V.sub.q71e =(V.sub.VREF11 +V.sub.VCONT)/2.

For example, when the voltage V_(VREF11) is 1 volts and the voltageV_(CONT) is made to vary from 0 to 2 volts, the emitter voltage V_(q71e)of the transistor Q₇₁ varies from 0.5 to 1.5 volts.

Generally speaking, in a laser printer or the like, a case will now beconsidered where scanning exposure is performed on a photosensitiveelement or the like through a polygon mirror or the like by light outputof a the semiconductor laser 1. In such a case, due to a change ofdistance to the photosensitive element and a change of shape of a beamspot formed on the photosensitive element and so forth during scanningexposure, known shading occurs. In order to perform a correctionoperation so as to eliminate influence of shading (shading correction),it is necessary to slightly adjust light output of the semiconductorlaser dynamically, or to perform a slight adjustment when lightintensity is set. FIG. 33A shows a light output waveform at an initialstate. FIG. 33B, 33C show light output waveforms after the current ofthe light emission instruction signal generating unit 222 is changed.FIG. 33B shows a case where only the current of the light emissioninstruction signal generating unit 222 is increased. In contrast tothis, FIG. 33C shows a case where the current of the light emissioninstruction signal generating unit 222 and the current of the addingcurrent setting unit 234 are increased together. In each case, a desiredlight output is obtained through control by the control system (negativefeedback loop 3) finally as a steady state output. However, at a risingtime, in the case of FIG. 33B, light output does not rise sharply. Incontrast to this, in the case of FIG. 33C, light output rises sharply.In a case where a desired light output is lower than the original lightoutput, when only the current of the light emission instruction signalgenerating unit 222 is decreased, light output sharply rises at firstand overshooting occurs due to the current of the adding current settingunit 234, and then, light output decreased to be the desired output.When both the current of the light emission instruction signalgenerating unit 222 and adding current setting unit 234 are decreasedtogether, such overshooting can be prevented. In the embodiment of thepresent invention, both the current of the light emission instructionsignal generating unit 222 and adding current setting unit 234 arechanged together. Specifically, as mentioned above, by changing thecontrol voltage of the VCONT terminal (shown in FIG. 16), the emittervoltage of the transistor Q₇₁ is changed, and thus the reference currentof the light emission instruction signal generating unit 222 and thereference current of the adding current setting unit 234 areincreased/decreased together. Thereby, when the voltage of the VCONTterminal is changed, a waveform, similar to a rectangular wave, such asthat shown in FIG. 33C can be obtained. Thus, the amount of controlperformed by the control system (negative feedback loop 3) can bereduced, and a quick response can be achieved. Such light output controlis performed for the above-mentioned shading correction and the slightadjustment of the semiconductor laser 1 light output for other purposes.

The above-mentioned second starting-up unit 35b will now be described,in relation to the light emission instruction signal generating unit222. As described above, the emitter voltage V_(q71e) of the transistorQ₇₁ (shown in FIG. 16) is an output of the voltage follower 244, and acapacitor C₃ controls control speed and stability of the output.However, if, when power supply is started, the power source rises fasterthan rising of the voltage follower 244, a setting operation in theaddition current setting unit 234 and so forth is performed before theemitter voltage of the transistor Q₇₁ reaches a predetermined voltage.If so, the semiconductor laser 1 may not provide a predetermined lightoutput. The second starting-up unit 235b acts to solve this problem.Specifically, the second starting-up unit 235b, similar to the firststarting-up unit 235a, prevents the timing generating unit 231 fromstarting through the TDSTART terminals and VPTDSTART terminal, until theemitter voltage of the transistor Q₇₁, that is, the voltage of a VRterminal exceeds a predetermined set voltage and thus the voltagefollower 244 is made operational. Only when the emitter voltage of thetransistor Q₇₁ reaches the predetermined set voltage, the secondstarting-up unit 235b allows the timing generating unit 231 to startthrough the TDSTART terminals and VPTDSTART terminal. In the starting-upunit 235, the first starting-up unit 235a and second starting-up unit235b are connected to be logic AND connection. Thereby, only when boththe power source voltage Vcc and the current of the light emissioninstruction signal generating unit 222 are in predetermined conditions,initialization and operations of all the electronic circuits arestarted.

The light emission instruction signal generating unit 222 will now bedescribed. The light emission instruction signal generating unit 222includes two 5-bit digital to analog converters connected with oneanother in parallel. One of the two 5-bit digital to analog convertersis shown in FIG. 17 and the other is not shown in the figures. The lightemission instruction signal generating unit 222 further includes acurrent addition driving unit, a current compensation unit for the lightemission instruction signal generating unit 222 and an offset currentgenerating unit. Each of the 5-bit digital to analog converter issimilar to the 5-bit digital to analog converter of the light emissioninstruction signal generating unit 222 in the second embodiment.Similarly, the current addition driving unit is the same as the currentaddition driving unit in the light emission instruction signalgenerating unit 222 in the second embodiment. Duplicated descriptionwill be omitted.

The two 5-bit digital to analog converters have circuit arrangementsalmost the same as one another. Why the two similar digital to analogconverters are provided will now be described. In principle, it isenough to provide only one digital to analog converter. However, byproviding two digital to analog converter in parallel, the lightemission instruction signal generating unit 222 can be applied forvarious types of light reception devices 2, the monitor currents PD ofwhich vary, and/or for various types of semiconductor lasers 1. A casewill now be considered where only one digital to analog converter isprovided and the current of the light emission signal generating unit222 is changed in a wide range for application to various types of lightreception devices 2 and/or various types of semiconductor lasers 1. Insuch a case, the linearity of the digital to analog converter may bedegraded, and/or, an error current may flow. By providing two digital toanalog converters, it is possible to reduce a range for which thecurrent of each digital to analog converter should be changed. One ofthe two digital to analog converters externally outputs the minimumvoltage, which determines current sources of the digital to analogconverter, via a DA1GND terminal. By causing the DA1GND terminal not tobe externally connected, the one digital to analog converter does notwork. When it is necessary to have a wider dynamic range, it is possibleto provide more than two digital to analog converters in parallel.

The current compensation unit for the light emission instruction signalgenerating unit 222 will now be described. In the above-described basecurrent compensation, a current is added for compensating base currentswhich are subtracted from the current I_(DA1). However, in the currentcompensation unit for the light emission instruction signal generatingunit 222, a current which is added to the current I_(DA1) is canceled.In an example of a transistor Q₁ shown in FIG. 15, the transistor Q₁draws the base current, and thus, the base current is added to thecurrent I_(DA1). In order to cancel the added current, a current mirrorcircuit 581 including transistors Q₈₆, Q₈₇ creates a current which isidentical to the base current of a transistor Q₈₅, and the createdcurrent is caused to flow to the base (PD terminal) of the transistorQ₁. In fact, the emitter current of the transistor Q₁ is equal to thecollector current of the transistor Q₈₅ which forms a current source,and therefore the base current of the transistor Q₈₅ is approximatelythe same as the base current of the transistor Q₁. As a result, byproviding current approximately the same as the base current of thetransistor Q₁ to flow to the base of the transistor Q₁, the base currentof the transistor Q₁ added to the current I_(DA1) is approximatelycanceled, and thus compensated.

The current amplification factor of an NPN transistor is referred to ash_(fen), the current amplification factor of an PNP transistor isreferred to as h_(fep), the base current of the transistor Q₁ isreferred to as i_(b) and the collector current of the transistor Q₈₇ isreferred to as I. Then, the emitter current i₁ of the transistor Q₁ isexpressed as follows:

    i.sub.1 =(1+h.sub.fen)·i.sub.b.

The base current i₂ of the transistor Q₈₅ is expressed as follows:

    i.sub.2 =(1+h.sub.fen)·i.sub.b /h.sub.fen.

The current passes through the current mirror circuit 581, and thus thecollector current I of the transistor Q₈₇ is expressed as follows:

    I=i.sub.b /{1+(2/h.sub.fep)}.

For example, when the current amplification factor h_(fep) is 100,I≈0.98i_(b). Current error of the current I_(DA1) in a case where nocompensation is performed is i_(b). Therefore, the error is 1/50 as aresult of the compensation being performed. By providing a similarcircuit arrangement, the base current of a transistor Q₈₃ can besimilarly compensated.

When it is necessary to improve the accuracy of this compensation, byusing a base current compensated current mirror circuit,

    I=i.sub.b /{1+(2/h.sub.fep.sup.2)}.

Thereby, the error is further decreased by the factor 1/50 (in the casewhere h_(fep) is 100).

The above-mentioned offset current generating unit will now bedescribed. As described above, in order to control light output of thesemiconductor laser 1 in real time in the negative feedback loop 3, itis not possible to cause light output of the semiconductor laser 1 to becompletely 0. Therefore, it is necessary to set a minimum value of lightoutput of the semiconductor laser 1. The offset current generating unitsets the minimum value of light output of the semiconductor laser 1. Theoffset current generating unit 583 is provided in a light emissioninstruction signal generating unit 222' as shown in FIG. 34 which is avariant of the above-described light emission instruction signalgenerating unit 222 and will be described later. Another offset currentsetting unit 582 is provided in the power source unit 561 shown in FIG.31. An offset current generated by these offset current generating units582, 583 is compared with a monitor current of the light receptiondevice 2 at the PD terminal, and thus a forward current of thesemiconductor laser 1 is generated through the error amplifier 223. Bythe forward current, an offset light emission intensity of thesemiconductor laser 1 can be set.

The offset current generating unit 582 includes a transistor Q₅₆ and aresistor R₂₅, and the emitter voltage of the transistor Q₅₆ is a stablevoltage in the integrated circuit 220 (LSI circuit) as described in thedescriptions of the power source unit 561. By providing the resistor R₂₅as an externally connected resistor or a variable resistor, it ispossible to externally set a desired current.

The offset current generating unit 583 in the light emission instructionsignal generating unit 222' includes a transistor Q₈₈ and a resistorR₅₁. By providing the resistor R₅₁ as an externally connected resistoror a variable resistor, it is possible to externally set a desiredcurrent. The base voltage of the transistor Q₈₈ is a voltage which ispreviously set through current setting means (specifically, as describedabove, the emitter voltage of the transistor Q₇₁ and the resistor R₄₁shown in FIG. 16) of the light emission instruction signal generatingunit 222 to be appropriate to the monitor current of the light receptiondevice 2 and so forth. Thereby, in a case where the light receptiondevice 2 of a larger monitor current is used, an offset currentgenerated by the offset current generating unit 583 automaticallybecomes larger. In a case where the light reception device 2 of asmaller monitor current is used, an offset current generated by theoffset current generating unit 583 automatically becomes smaller. Thus,the offset current is automatically set according to a previously setcondition of the above-mentioned current setting means of the lightemission instruction signal generating unit 222. Thus, by setting themaximum current of the light emission instruction signal generating unit222, the offset current is automatically set.

The final offset current is obtained as the total of the offset currentsgenerated by the offset current generating units 582 and 583. Theresistance values of the external resistors R₂₅, R₅₁ may be previouslyset to adequate values. Then, it is not necessary to change the setresistance values even if monitor current characteristics of the lightreception device 2 varies. As described above, the offset current isautomatically set corresponding to the set condition of theabove-mentioned current setting means of the light emission instructionsignal generating unit 222, thus an appropriate offset current can beobtained and appropriate offset light output of the semiconductor laser1 can be obtained, automatically. Thus, an adjustment process can beperformed automatically.

In the embodiment, the light emission instruction signal generating unit222 includes the two digital to analog converters connected in parallel.However, it is also possible that certain components, which are includedin each of the two digital to analog converters are to be commonly used.Thus, the variant 222' of the above-described light emission instructionsignal generating unit 222, shown in FIG. 34, is obtained. In thecircuit arrangement of the light emission instruction signal generatingunit (variant) 222' shown in FIG. 34, the certain components whichperform identical functions are commonly used. Thereby, the number ofcomponents can be effectively reduced.

In the fifth embodiment, almost all of the semiconductor laser controlsystem 213 is formed as the integrated circuit 220 using bipolartransistors. It is also possible that the system is formed as anintegrated circuit using C-MOS transistors. It is also possible that thesystem is formed as an integrated circuit using a combination of bipolartransistors and C-MOS transistors. It is also possible that the systemis formed as a non-integrated circuit.

A sixth embodiment of the present invention will now be described withreference to the drawings. FIG. 35 shows an example of a detailedarrangement of the semiconductor laser control system in the sixthembodiment. The sixth embodiment is similar to the fifth embodimentdescribed above. The same reference numerals are given to blocks of thesixth embodiment which are substantially identical to those of the fifthembodiment. Duplicated descriptions will be omitted.

As common characteristics of a semiconductor laser such as thesemiconductor laser 1, there is a change of the operation current due toa change of temperature, a change (degradation) of the operation currentand the differential quantum efficiency due to elapsing time. FIG. 9shows a change (degradation) of the operation current and thedifferential quantum efficiency due to elapsing time. When the time t=0,the operation current Iop(0) is required for driving the semiconductorlaser 1 and causing the semiconductor laser 1 to emit a light output P0.When the time t=t0, that is, after the time t0 has elapsed, theoperation current Iop(t0) is required for driving the semiconductorlaser 1 and causing the semiconductor laser 1 to emit the same lightoutput P0. With regard to a change of the operation current, by causingthe negative feedback loop 3 to always operate, even if the oscillationthreshold current Ith changes due to a change of temperature, thecontrol system (negative feedback loop 3 follows the change, and thecontrol system always causes the oscillation threshold current to flowthrough the semiconductor laser 1 as a forward current. In order for thenegative feedback loop 3 to always operate, light output of thesemiconductor laser 1 is not completely 0. It is necessary for thesemiconductor laser 1 to provide a slight light output, that is, toperform offset light emission. Actually, light output of thesemiconductor laser is controlled between a set maximum light emissionand the offset light emission by the negative feedback loop 3.

When the semiconductor laser 1 is degraded as shown in FIG. 9 due toelapse of time , in a case where the degradation is not serious, thedegradation is detected and an appropriate current setting is performedto compensate the degradation. Such an operation was described above asthe detection of the differential quantum efficiency and voltageshifting amount is set according to a result of the detection, withreference to FIG. 24 (time charts). However, if the degradation isserious and an excessive current flows through the semiconductor laser 1driving circuit, it is necessary to stop operation of the semiconductorlaser control system so as to protect the integrated circuit 220 (LSIcircuit). For this purpose, a semiconductor laser degradation detectingunit 662 is provided as shown in FIG. 35. FIG. 36 shows an example of acircuit arrangement using bipolar transistors in the semiconductor laserdegradation detecting unit 662. In this circuit arrangement, a voltageVLD of the LD terminal is always monitored, and when the voltage VLDexceeds a certain reference voltage, an error signal is output from anerror terminal LDERR. In the circuit shown in the figure, transistorsQ₅₇, Q₅₈ form a differential switch 664. The certain reference voltagewhich is applied to the base of the transistor Q₅₈ is generated in thepower source unit 561 shown in FIG. 31. When the voltage VLD of the LDterminal exceeds the reference voltage, the transistor Q₅₈ turns on, andas a result, a current is drawn to the collector of a transistor Q₅₉ viaan LDERR terminal. Thus, an open collector output is provided.

When the semiconductor laser 1 is degraded or some other trouble occurstherein, the semiconductor laser 1 provides excess light output, and thevoltage VLD of the LD terminal excessively increases. The semiconductorlaser degradation detecting unit 662 detects the increased voltage andoutputs the error signal. Thereby, the semiconductor laser controlsystem is not used in this condition, and further serious trouble can beprevented from occurring.

In the starting-up circuit 235 shown in FIG. 32, the base of atransistor Q₆₄ which is connected to the base of a transistor Q₆₁ isconnected to a reset terminal RESET. The transistor Q₆₄ turns on or offaccording to a reset signal which is a control signal and is given tothe reset terminal RESET externally. When the reset signal is given tothe reset terminal REST, the transistor Q₆₄ turns on, and, theabove-described starting up operation of the starting-up unit 235a isdiscontinued, and the initialization operation of the integrated circuit620 is forcibly performed. Thereby, the initialization operation of theintegrated circuit 620 can be performed at any time. Therefore,protection of the semiconductor laser 1 can be positively performed.

In the embodiment, the current driving unit 224 is incorporated in theerror amplifier 223 provided in the path of the negative feedback loop3, as the voltage shifting unit 225, as shown in FIG. 15. However, asshown in FIG. 37, it is also possible to provide the current drivingunit 224 in a path separate from the negative feedback loop 3.

Further, in the sixth embodiment, almost all of the semiconductor lasercontrol system 613 is formed as an integrated circuit 620 using bipolartransistors. It is also possible that the system is formed as anintegrated circuit using C-MOS transistors. It is also possible that thesystem is formed as an integrated circuit using a combination of bipolartransistors and C-MOS transistors. It is also possible that the systemis formed as a non-integrated circuit.

A semiconductor laser control system in a seventh embodiment of thepresent invention is used, in a laser printer or the like, as a controlsystem including the negative feedback loop which controls light outputof the semiconductor laser, the light output being used for opticalwriting. A basic configuration and function of the semiconductor lasercontrol system is the same as the semiconductor laser control system inthe related art shown in FIG. 3. Image data and pixel clock signal areinput to the pulse width generating unit and data modulation unit 11which outputs a light emission instruction signal. The light receptiondevice 2 monitors light output of the semiconductor laser 1. Thesemiconductor laser 1 and light reception device 2 are connected withthe semiconductor laser control unit and semiconductor laser drivingunit 12. The light emission instruction signal generated by the pulsewidth generating unit and data modulation unit 11 is given to thesemiconductor laser control unit and semiconductor laser driving unit12.

As in the above-described embodiments, the pulse width and intensitycombined modulation method is used for obtain multiple tone levels inone dot. Specifically, according to input image data, the pulse widthgenerating unit and data modulation unit 11 performs basically the PWMmethod and also performs the PM method for compensating for a change ofpulse width.

The basic concept of the pulse width and intensity combined modulationmethod was described above with reference to FIGS. 4, 5A and 5B.Duplicated descriptions will be omitted.

An example of an arrangement, functions and advantages of thesemiconductor laser control unit and semiconductor laser driving unit 12are substantially the same as those which were described with referenceto FIGS. 1, 2A and 2B. Duplicated descriptions thereof will be omitted.

FIG. 38 shows a specific block arrangement of the semiconductor lasercontrol system in the seventh embodiment. The semiconductor lasercontrol system in the seventh embodiment is similar to the semiconductorlaser control system in the first embodiment. In fact, the blockarrangement of the seventh embodiment shown in FIG. 38 is similar to theblock arrangement of the first embodiment shown in FIG. 10. The samereference numerals are given to components which correspond to those ofthe first embodiment. Duplicated descriptions will be described.

The specific arrangement and functions of the pulse width generatingunit and data modulation unit 11 are substantially the same as those ofthe first embodiment. Duplicated descriptions will be omitted.

The arrangement and functions of the light emission instruction signalgenerating unit 112 are substantially the same as those of the lightemission instruction signal generating unit 422 of the fourthembodiment. Descriptions thereof were provided with reference to FIGS.29A, 29B and 29C. Duplicated descriptions will be omitted.

The pulse width modulation and intensity modulation signal generatingunit 11 includes a data converting unit 116 which is data convertingmeans, a pulse width modulation unit 117 which is pulse width modulatingmeans and a pulse generating unit 118 which includes PLL and is pulsegenerating means. The data converting unit 116, pulse width modulationunit 117 and pulse generating unit 118 are substantially the same as thedata converting unit 463, pulse width modulation unit 464 and pulsegenerating unit 462 of the fourth embodiment shown in FIG. 28.Duplicated descriptions will be omitted. The pulse generating unit 118includes a phase frequency comparator (PD) 624, a voltage controlledoscillator (VCO) 725 and a lowpass filter (LPF) 726.

Logic of obtaining a light output waveform such as that shown in FIG.5A, for example, is the same as described above with reference to theequations (1), (2) and (3) in the descriptions of the first embodiment.Duplicated descriptions will be omitted.

Specific arrangements and functions of the data converting unit 116 andpulse width modulation unit 117 are substantially the same as those ofthe data converting unit 116 and pulse width modulation unit 117 in thefirst embodiment shown in FIG. 12A. Duplicated descriptions will beomitted.

The pulse width modulation unit 117 can be formed by a combination ofAND, OR gates which perform logic operations on a plurality of pulsesprovided by the pulse generating unit 118, and can be easily formed by alogic circuit arrangement.

For the sake of simplification of equations, for example, the firstequation of the equations (2) is expressed as the following equation(4): ##EQU3## Further, x_(n), X_(m), X_(n) ', X_(m) ' may be produced asfollows: ##EQU4## Therein, X_(H) and X_(L) are signals which always havethe H level and L level, respectively. By substituting X_(H) or X_(L)for X₀, X₄ in the equations (2), linearity of the signal can beimproved. This is because, for example, when X₀ is selected as beingX_(n), the equations (1) include the terms, X₀ ·X₀. As a result, risingand decaying coincide. The resulting rising and decaying are delayed incomparison to other cases. Accordingly, when the clock signal is of ahigh frequency, linearity of produced pulse widths is degraded.

The above-shown equations (1), (2) and (3) indicate logic equations forobtaining a waveform in which a pulse width starts from the left sidesuch as that shown in FIG. 5A at the top. However, when the pulse widthmodulation unit 117 and the data converting unit 116 are configurated soas to perform a logic operation shown in the following equations (7),(8) and (9), one of a waveform in which a pulse width starts from theleft side (such as that shown in FIG. 5A) and a waveform in which apulse width starts from the right side (such as that shown in FIG. 5B)can be selected by using input position control data P. ##EQU5## Thus,dot position control (that is, whether a waveform in which a pulse widthstarts from the left side (such as that shown in FIG. 5A) or a waveformin which a pulse width starts from the right side (such as that shown inFIG. 5B) is selected) can be performed for each dot. By alternatelyrepeating a waveform in which a pulse width starts from the left sideand a waveform in which a pulse width starts from the right side, dotconcentration pulse width modulation can be performed. The pulsesPW_(on), PW_(da) have a relationship such that a pulse PW_(on) is alwaysshorter than a pulse PW_(da) by the minimum pulse width. Therefore, partof the modulation data is common and it is assumed that D_(ni) =D_(ni)', D_(mj) =D_(mj) '. Accordingly, for example, in FIG. 12A, the logicunits 28, 30 can be omitted, the number of components of the dataconverting unit 116 can be reduced, and the number of data lines to thepulse width modulation unit 117 can be reduced.

The arrangement of the pulse width modulation unit 117 will now beconsidered with reference to FIG. 12B which is obtained as a result ofpartially rewriting the arrangement of the unit 117. In the arrangementshown in FIG. 12B, the multiplexers 130a, 130c select pulses X_(n),X_(m) from pulses X_(i), X_(j) provided by the pulse generating unit 118based on pulse width modulation data D_(ni), D_(mj), respectively. Theoutput X_(n) from the multiplexer 126 and the internal clock X₀ areinput to the AND (logical multiplication) gate 130a. The output X_(m)from the multiplexer 128 and the inverted internal clock X₀ are input tothe AND (logic multiplication) gate 130c. The outputs of the AND gates130a, 130c are input to the OR (logic sum) gate 130e. A number n of sucharrangement units are provided. In the case of FIG. 12A, 2 sucharrangements are provided. In the pulse width modulation unit 117 inwhich such an arrangement unit is used as a base, X_(n), X_(m) areselected independently. Therefore, the circuit arrangement the same asthat used in the case of an equal mode (described later) can also beused in the case of double mode (described later). Thus, such anarrangement of the pulse width modulation unit 117 is advantageous.

In the seventh embodiment, output modes which have different writingfrequencies can be selected. That is, for example, for image data suchas that of a photographic image, which needs multiple tone levels forone dot, the above-described pulse width and intensity combinedmodulation method is used and writing is performed at a speed equal tothat of an input clock signal (such a writing or output mode will bereferred to as "equal mode"). However, for image data such as characterimage, which needs higher writing density than multiple tone levels forone dot, writing frequency is increased to, for example, double that ofan input clock signal (such a writing mode will be referred to as"double mode"). One of these two writing modes can be selected. For thispurpose, a frequency selecting signal M for selecting output modes isinput to the data converting unit 116 of the pulse width modulation andintensity modulation signal generating unit 111 shown in FIG. 12A. Thesignal M is equal to 1 when the pixel writing clock frequency is equalto the input clock frequency, and the signal M is equal to 0 when thepixel writing clock frequency is double that of the input clockfrequency.

FIGS. 39A, B and C show a basic concept of light output waveforms whenthe pixel writing clock frequency is changed. FIG. 39B shows an exampleof a light output waveform in the case where the pixel writing clockfrequency is the same as the input clock frequency (the period: T_(CK)).According to the above-described pulse width and intensity combinedmodulation method in one dot, pulse width modulation uses 3 bits, andintensity modulation uses 5 bits, and a 8-bit tone is obtained on theother hand, FIG. 39C shows an example of a light output waveform whenthe pixel writing clock frequency is double that of the input clockfrequency, that is, the period is 1/2. In this double mode, a 4-bit(pulse width modulation using 2 bits and intensity modulation using 2bit) tone for one dot is obtained. Thus, the number of the availabletone levels is reduced. However, image writing density in a mainscanning direction can be double. In this case, in addition to doublingthe pixel writing clock frequency, the scanning rate of a laser beamemitted by the semiconductor laser 1, for example, the rotation speed ofa polygon mirror is doubled, and the line velocity of a rotatingphotosensitive body is doubled. Thereby, the writing speed is increasedto be doubled.

As shown in the time charts FIGS. 39A, 39B, 39C, in the equal mode inwhich the pixel writing clock frequency is equal to the input clockfrequency, an N-bit tone (pulse width modulation using M bits andintensity modulation using (N-M) bits) for one dot is obtained. In thedouble mode in which the pixel writing clock frequency is double that ofthe input clock frequency, an N/2-bit tone (pulse width modulation usingM-1 bits and intensity modulation using (N/2-M+1) bits) for one dot isobtained. Thus, the number of image data input terminals can be thesame. That is, in the double mode, image data for two dots is input inparallel. Production by the converting unit 116 of modulation dataaccording to the frequency selecting signal M enables common use of thepulse width modulation unit 117 because the minimum pulse width is alsocommon as shown in FIGS. 39B, 39C. Thus, the above-described processingcan be achieved by use of the circuit shown in FIG. 12A, the frequencyselecting signal is input to the data converting unit 116 as mentionedabove, and, in the double mode, D_(ni), D_(mj) are produced asrespective dot modulation data.

It is assumed that the data converting unit 116 shown in FIG. 12A isconfigured to perform a logic operation of the equations (10) shownbelow, and thus the data converting unit 116 acts as mode change-overmeans. In the example of the equation (10), in the double mode, thefirst dot is written in accordance with the more significant four bits(D₇, D₆, D₅, D₄) of input image data D₇ to D₀ and the second dot iswritten in accordance with the less significant four bits (D₃, D₂, D₁,D₀). Further, intensity modulation data is obtained as a result ofperforming a logic operation in the equation (11) shown below. ##EQU6##

The pulse width modulation unit 117 (shown in FIG. 12A) can be commonlyused when the pixel writing clock frequency varies, and thus performsthe logic operation of the above-shown equations (7) and (8). Bysubstituting the equations (12), (13), shown below, for the equations(10), (11), even when the pixel writing clock frequency is doubled, oneof a waveform in which a pulse width starts from the left side and awaveform in which a pulse width starts from the right side is selectedfor each dot. ##EQU7## Specifically, when the pixel writing clockfrequency is doubled, one bit (in this case, D₇) of the more significantfour bits is used as position control (whether a waveform in which apulse width starts from the left side or a waveform in which a pulsewidth starts from the right side) data, and the remaining three bits areused for pulse width modulation (5-value output, 0 to 4). Similarly, onebit (in this case, D₃) of the less significant four bits is used asposition control data, and the remaining three bits are used for thepulse width modulation. Further, when M=0 (that is, when double mode isselected), all of the intensity modulation data is at the L level.

Generally, when an input data series is an N-bit image data series, thenumber of tone levels which can be output is 2^(N) Therefore, one orseveral levels are still needed to complete the 2^(N) +1 output states,0/2^(N) to 2^(N) /2^(N). When one bit is added as the position control(whether a waveform in which a pulse width starts from the left side ora waveform in which a pulse width starts from the right side) signal,2^(N) tone output is obtained for each modes of a mode in which a pulsewidth starts from the left side and a mode in which a pulse width startsfrom the right side. In this case, one state is still needed in eachmode to obtain the complete output states. In order to obtain 2^(N) +1tone completely, N+1 bits are needed as image data and one bit is neededas the position control signal. However, each of the full turning offoutput state (that is, one dot full turning off: 0/2^(N)) and the fullturning on output state (that is, one dot full turning on: 2^(N) /2^(N))is common between a waveform in which a pulse width starts from the leftside and a waveform in which a pulse width starts from the right side.Therefore, when the full OFF state, full ON state, states of a waveformin which a pulse width starts from the left side and a waveform in whicha pulse width starts from the right side of middle levels 1/2^(N) to(2^(N) -1)/2^(N) are output. The number of the states of a waveform inwhich a pulse width starts from the left side and a waveform in which apulse width starts from the right side of 1/2^(N) to (2^(N) -1)/2^(N) is2·(2^(N) -1). Accordingly, output of the total 2^(N+1) states enablesoutput of 2^(N) +1 tone output including the position control (whether awaveform in which a pulse width starts from the left side or a waveformin which a pulse width starts from the right side) from the data seriesof N+1 bits.

For example, the data series is assumed to be a 4-bit data series, andit is also assumed that 9-level tone is obtained. That is, nine values,0/8 to 8/8 are obtained, and a total of 16 states are obtained, 0/8(always turning off), 8/8 (always turning on), 1/8 to 7/8 for each of awaveforms in which a pulse width starts from the left side and awaveform in which a pulse width starts from the right side. In order toachieve this output, the following truth table is used.

    ______________________________________                                        D7    D6    D5    D4  OUTPUT   D7  D6  D5  D4  OUTPUT                         D3    D2    D1    D0  STATE    D3  D2  D1  D0  STATE                          ______________________________________                                        0     0     0     0   0        1   0   0   0   LEFT 1                         0     0     0     1   RIGHT 1  1   0   0   1   LEFT 2                         0     0     1     0   RIGHT 2  1   0   1   0   LEFT 3                         0     0     1     1   RIGHT 3  1   0   1   1   LEFT 4                         0     1     0     0   RIGHT 4  1   1   0   0   LEFT 5                         0     1     0     1   RIGHT 5  1   1   0   1   LEFT 6                         0     1     1     0   RIGHT 6  1   1   1   0   LEFT 7                         0     1     1     1   RIGHT 7  1   1   1   1   8                              ______________________________________                                    

Input of such data series and use of such a logic enables output of adesired tone level number with the input data series of the number ofbits which is short of one bit. Thereby, an input data transfer rate canbe reduced and the number of input terminals can be reduced. Further,the number of buffer memories which are generally provided before thedata converting unit 116 can be reduced. In another view point, when thenumber of input data lines is fixed, by using such data series and suchlogic, the tone level number can be effectively increased. This methodis advantageous especially when the number of bits allocated for one dotis not large.

Specifically, a case will now be considered, where the pixel writingclock frequency is doubled, and the input data series includes the moresignificant four bits and the less significant four bits which representa 9-level tone including dot position control (whether a waveform inwhich a pulse width starts from the left side or a waveform in which apulse width starts from the right side) for each dot (see theabove-shown truth table). In such a case, without increasing the numberof input data lines, the number of tone levels can be increased and thushigh quality images can be obtained.

For this purpose, the equations (14), (15), (16) shown below aresubstituted for the above-shown equations (7), (12), (13). In theequations (14), X_(n), X_(n) ', X_(m), X_(m) ' are in accordance withthe above-shown equations (8). Further, when M=0, all of the intensitymodulation data D_(pk) is at the L level except D_(p4) which is at the Hlevel. ##EQU8##

FIG. 40 shows examples of waveforms of light outputs and PW_(on),PW_(da) corresponding to the logic of the above-shown truth table. Inthe seventh embodiment, as shown in FIG. 40, in the double mode,according to input image data, PW_(da) is either a pulse having a pulsewidth the same as a pulse width of PW_(on) or a pulse having a pulsewidth longer than a pulse width of PW_(on) by the minimum pulse width,and intensity modulation data is fixed to be half the maximum lightoutput intensity. Then, when only the pulse PW_(da) is at the H leveland the pulse PW_(on) is at the L level, intensity modulation isperformed, that is, the light intensity is halved as shown in thefigure. Thereby, it is not necessary to change intensity modulation datafor each half of an input clock pulse as in the above-shown equations(11). Thus, it is possible to achieve high-speed data writing. In theexample of FIG. 40, PW_(da) is always a pulse having a pulse widthlonger than a pulse width of PW_(on) by the minimum pulse width. Whenonly the pulse PW_(da) is at the H level and the pulse PW_(on) is at theL level but the PMD (power modulation data) is 0, the output is 0. Forexample, in the case of RIGHT2 where PW_(da) is longer than PW_(on),when only the pulse PW_(da) is at the H level and the pulse PW_(on) isat the L level but the PMD (power modulation data) is 0, the output is0.

An eighth embodiment of the present invention will now be described. Theeighth embodiment is similar to the above-described seventh embodiment.Duplicated descriptions will be omitted.

In the eighth embodiment, as shown in FIG. 41, a switch unit 841 whichfunctions as part of the output mode change-over means is insertedbetween the pulse width modulation unit 117 and light emissioninstruction signal generating unit 112. The switch unit 841 performschange-over operations according to a forcible light cessationinstruction signal S_(W1) and a forcible light emission instructionsignal S_(W2) Specifically, the pulse width modulation unit 117 andswitch unit 841 perform logic operations of the logic equations (17),shown below, instead of the logic equations (14). ##EQU9## Thereby,without regard to input image data, the semiconductor laser 1 isforcibly turned off or forcibly turned on. It is noted that there is nocase where the forcible light cessation instruction signal S_(W1) andforcible light emission instruction signal S_(W2) are at the H levelsimultaneously.

According to the arrangement shown in FIG. 41, when the forcible lightcessation instruction signal S_(W1) is ON (is at the H level), all ofthe pulse width modulation output is OFF. This condition is equivalentto the condition where all of the image data is 0. Therefore, when datawriting is not needed continuously, the forcible light cessationinstruction signal S_(W1) is caused to be ON, and it is not necessary tocause all of image data to be 0. Thus, in such a case, control is easy.On the other hand, when all of the image data is 0 and the forciblylight emission instruction signal S_(W1) is ON, light emission can beperformed in any manner in a full-turning-on pulse width modulationmethod in a frequency of the forcibly light emission instruction signalS_(W1) which has no relation to the input clock frequency. Such lightemission control can be effectively used for generating a detect pulseor the like.

FIG. 42 shows an example of a block arrangement of the data convertingunit 116, pulse width modulation unit 117 and switch unit 841 which areconfigured to perform pulse width modulation in accordance with thelogic equations (17), (8), (15). The data converting unit 116 includestwo logic units 842, 843 which perform the logic operations of theequations (15) and produce pulse width modulation data based on inputimage data D₀, D₁, . . . , D₇, position control data P and frequencyselecting signal M. The output sides of the logic units are connectedwith means for temporarily holding the produced data, for example, latchcircuits 844, 845. A gate signal generating circuit 846, which generatesa gate signal based on output of the pulse generating unit 118, isconnected to the latch circuits 844, 845.

The pulse width modulation unit 117 includes multiplexers 847, 848, 849and 850. Four (X_(i)) of different phase pulses X₀, X₁, . . . , X₇ areinput to the first multiplexer 847, which selects one or inverted one ofthe input pulses X_(i), a constant H level signal, or a constant L levelsignal, in accordance with the pulse width modulation signals D_(n1),D_(n2), D_(n3), D_(n4) which act as selecting signals. The othermultiplexers 848, 849, 850 have similar functions, as shown in FIG. 42.Multiplexers 851, 852 are connected to the output sides of themultiplexers 847, 848, 849, 850, as shown in the figure. The multiplexer851 selects one of X_(n), X_(n) ' which are outputs of the multiplexers847, 848 in accordance with the pulse width modulation signals D_(n5),D_(n6) which act as selecting signals. The multiplexer 851 has a similarfunction as shown in the figure. An internal clock generating unit 853generates an internal clock signal, and outputs X₀ as it is or via abuffer. AND gates 854a, 854b, 854c, 854d, OR gates 854e, 854f areprovided, which generate pulses PW_(da), PW_(on) according to the logicequations (17) using the outputs of the multiplexers 851, 852 and theinternal clock signal from the internal clock generating unit 853.Multiplexers 855, 856 which form the switch unit 841 are inserted to theoutput sides of the OR gates 854e, 854f. The multiplexers 855, 856performs change-over operations and output the outputs of the OR gatesas they are, outputs the constant L level signal or outputs the constantH level signal.

The data converting unit 116, pulse width modulation unit 117 and switchunit 841 can be easily formed as an integrated circuit by bipolartransistor or the like. FIG. 43 shows an example of a circuitarrangement of the latch circuit 44 which is an example of data holdingmeans which holds input image data or modulation data. When input datais D, D (differential inputs), and held data is Q, Q.

    Q=DG+QG.

When the latch gate signal G is at the H level, the input signal D isoutput. When the latch gate signal is at the L level, the previous datais held. The latch gate signal G can be easily generated through thegate signal generating unit 846 based on pulses generated by the pulsegenerating unit 118 or using a combination of these pulses. For example,with reference to the timing charts of FIG. 13, a latch gate signal G₁which holds the modulation data D_(n) for selecting X_(n) is obtained asfollows: G₁ =X₂ ·X₄. A latch gate signal G₂ which holds the modulationdata D_(m) for selecting X_(m) is obtained as follows: G₂ =X₆ ·X₀.

Further, two latch circuits, each circuit being the latch circuit 844such as that shown in FIG. 43, are connected in a cascade connection,and a latch gate signal for the second latch circuit is a signal whichis obtained as a result of inverting the latch gate signal for the firstlatch circuit or a H level signal for a certain fixed period within aperiod during which the latch gate signal for the first latch circuit isat the L level. Thus, a flip-flop is formed. When the data holding meansis formed by a flip-flop, data present immediately before decaying ofthe latch gate signal of the first latch circuit is held for one clockperiod. However, only by the latch circuit 44, when input data varieswhile the latch gate signal is at the H level, output also varies.Therefore, the data holding means formed by a flip-flop is suitable asdata holding means for holding intensity modulation data.

FIG. 44 shows an example of a logic circuit 857 which is formed bybipolar transistors, is part of the logic unit 842 and is configured toperform the first equation for D_(n1) in the equations (15). An outputof the logic circuit 857 may be held by the latch circuit 844 such asthat shown in FIG. 43.

However, as shown in FIG. 45, it is also possible to form a logiccircuit 858 which generates the pulse width modulation data D_(n1) andalso holds it. Thereby, the number of components can be effectivelyreduced. The logic circuit 858 shown in FIG. 45 performs the logicoperation of the following equation (18): ##EQU10## In FIG. 45, G1represents a latch gate signal. Further, V_(th1), V_(th2) representthreshold voltages of respective logic levels, respectively. Further,input signals such as D₅ are obtained as a result of convertingexternally input data such as D_(5in) into internal level signals, forexample, through a level shift circuit 859 such as shown in FIG. 46.Such level shift circuits perform voltage shift using emitter followers,diodes, resistors, and so forth, appropriately.

The frequency selecting signal M, M are generated from an externalfrequency selecting signal mode through a selecting signal generatingcircuit 860 such as that shown in FIG. 47. In FIG. 47, a transistor Q₁having a reference voltage V_(BBp) applied to its base and a resistor R₁forms a constant-current source 861 for providing a current I₁.Transistors Q₂, Q₃ form a differential switch 862. The externalfrequency selecting signal mode is converted into an internal levelsignal and then applied to the base of the transistor Q₂. Transistor Q₄,Q₅, Q₆, Q₇, resistors R₄, R₅, R₆ generate a threshold voltage which isapplied to the base of the transistor Q₃. When the external frequencyselecting signal mode is at the H level, the transistor Q₃ turns on andthe collector current thereof is the current I₁. Thus, the voltage ofthe selecting signal M is I₁ ·R₁ +V_(BE), where V_(BE) is thebase-emitter voltage of a transistor, and the selecting signal M is inthe ON state. On the other hand, because the collector current of thetransistor Q₂ is approximately 0, the selecting signal M is in the OFFstate. When the external frequency selecting signal is at the L level,the opposite operation is performed. When the selecting signals M, M areapplied to the bases of transistors of a current switch (for example, acurrent switch 863 shown in FIG. 45), the collector current of eitherone of the transistors flows.

For the other equations of the equations (15), logic circuits whichperform the logic operations of the equations can be formed by bipolartransistors. For the other logic equations, logic circuits which performthe logic operations of the equations can be formed as an integratedcircuit by bipolar transistors. For example, in the case of the firstequation of the equations (9), a current source is used instead of thecurrent switch 863 and a circuit 864 which is located at a higher levelis removed.

For purpose of obtaining intensity modulation data D_(PK), latchcircuits are connected cascade. FIG. 48 shows an example of a circuitarrangement of a D_(p4) generating unit 866 for obtaining D_(p4) of thefirst equation of the equations (16). In a second latch circuit 868 oftwo latch circuits 867, 868, configurations of data holding and alsodata generating logic operation are incorporated. The first latchcircuit 867 is similar to the circuit arrangement shown in FIG. 43 butis configured so as to provide only a non-inverted output. Thus, thenumber of components can be reduced. In FIG. 48, D₄ is obtained as aresult of being converted into an internal level signal through a levelshift circuit such as that shown in FIG. 46. V_(th1) represents athreshold voltage. M and M are generated by the circuit shown in FIG.47. G₁ and G₃ are respective latch gate signals. G₁ was described above.G₃ =X₀. In FIG. 48, by connecting the collector of a transistor Q₁₀ to aresistor R₇, D_(p3), D_(p2), D_(p1), D_(p0) in the equations (16) can begenerated.

The pulse width modulation unit 117 shown in FIG. 42 can be formed asshown in FIGS. 49, 50 by bipolar transistors. FIG. 49 shows a circuitwhich performs the logic operation of the first equation of theequations (8), and corresponds to the multiplexer 847 shown in FIG. 42.FIG. 50 shows a circuit which performs the logic operation of the firstequation of the equations (17), and corresponds to the multiplexer 851,852, 855, AND gates 854a, 854c, OR gates 854e shown in FIG. 42.

In FIG. 49, a transistor Q₁₁ having a reference voltage V_(BB) appliedto the base thereof and a resistor R₈ form a current source 869 whichprovides a current I. There are differential switches 870, 871, 872. Oneof the transistors of the differential switches 870, 871 turns on bypulse width modulation data D_(n1), D_(n2). Thereby, current flowsthrough one of differential switches 873, 874, 845, 876, to which thecollector of the transistor, which turns on, is connected. Phasedifferent pulses which are generated by the pulse generating unit 118are applied to the differential switches 873, 874, 875, 876. In each ofthe differential switches 873, 874, 875, 876, the right side transistorhas a pulse X_(i) (i=1, 2, 3, 4, from the left, as shown in the figure)applied thereto, and the left side transistor has the inverted signalthereof. It is also possible that a certain fixed voltage is applied tothe left side transistor. However, by using differential inputs as shownin the figure, a swing voltage required for switching can be effectivelyreduced. Therefore, in a case such as that of FIG. 49 in whichtransistors are stacked in multiple layers, use of differential inputsare advantageous.

In inputting of X_(i), linearity of pulse width modulation data to begenerated is improved. For example, a case where D_(n2) =0, D_(n1) =1will now be described. In this case, the right side transistor of thedifferential switch 870 turns on and a current flows through thedifferential switch 874, and no current flows through each of the otherthree differential switches 843, 875, 876. Thus, the pulse X₆ isselected. When X₆ is at the H level, a current flows through adifferential switch 877. When X₆ is at the L level, a current flowsthrough a differential switch 878. Respective pulse width modulationdata D_(n3) and D_(n4) is applied to the differential switches 877 and878. When both D_(n3) and D_(n4) are at the H level, a terminal voltageof a resistor R₉ is a signal equal to X₆. When both D_(n3) and D_(n4)are at the L level, the terminal voltage of the resistor R₉ is a signalequal to X₂ (the inverted signal of X₆). When D_(n3) =0, D_(n4) =1, theterminal voltage of the resistor R₉ is always at the L level withoutregard to X₆. When D_(n3) =1, D_(n4) =0, the terminal voltage of theresistor R₉ is always at the H level without regard to X₆. The signalbecomes X_(n) via an emitter follower and a diode. Similarly, theinverted signal X_(n) is generated. For X_(n) ', X_(m), X_(m) ',circuits for generating the signals can be obtained by appropriatelychanging input signals in accordance with the equations (8). Further,circuits for generating of X_(n) through the other equations can also beobtained in a similar manner.

A circuit arrangement shown in FIG. 50 is similar to the circuitarrangement shown in FIG. 49. Therefore, descriptions thereof will besimply presented. An internal clock signal CK₀ is obtained as a resultof performing voltage shift on X₀. For correspondence with theabove-shown logic equations, the internal clock signal will be referredto as X₀. When X₀ is at the H level, a current flows through the leftside transistor of a differential switch 872a. Then, a current resultingfrom logic multiplication (AND) of X_(n) and X₀ flows through adifferential switch 877a when D_(n5) =0. A current resulting from logicmultiplication (AND) of X_(n) ' and X₀ flows through a differentialswitch 877a when D_(n5) =1. When X₀ is at the L level, in accordancewith D_(m5), a current resulting from logic multiplication (AND) ofX_(m) and X₀ or X_(m) ' and X₀ flows through the differential switch877a. Therefore, a current resulting from logic sum (OR) thereof flowsthrough the differential switch 877a. The inverted current thereof flowsthrough a differential switch 878a. Then, when both the forcible lightcessation instruction signal S_(W1) and forcible light emissioninstruction signal S_(W2) are at the L level, the logic sum signalbecomes a terminal voltage of a resistor R₃ ', and then becomes PW_(da)via an emitter follower. When only the forcible light cessationinstruction signal S_(W1) is at the H level, PW_(da) is always at the Llevel without regard to pulse width modulation data. In this case,PW_(on) is also always at the L level. Thereby, the semiconductor laser1 is forcibly turned off. When only the forcible light emissioninstruction signal S_(W2) is at the H level, PW_(da) is always at the Hlevel. In this case, the semiconductor laser 1 is forcibly turned on. Acircuit for generating PW_(on) can be obtained as a result ofappropriately changing the input signals in the circuit arrangementshown in FIG. 50.

With reference to FIG. 1, functions of the semiconductor laser controlunit and semiconductor laser driving unit 12 will be described inconnection with the arrangement of the pulse width generating unit anddata modulation unit 11, in outline. The constant-current source 5 inthe negative feedback loop 3 provides a current generated in accordancewith modulated data (pulse width modulated two pulses PW_(on), PW_(on)and intensity modulated data PMDATA). This current is compared with themonitor current which is output by the light reception device 2 and isproportional to light output of the semiconductor laser 1. An errorresulting from the comparison is converted into a forward current of thesemiconductor laser 1 through the error amplifier 6 and the transistor7. Thus, the negative feedback loop 3 is formed. The current source 4provides a current which is generated in accordance with the two pulsewidth modulated pulses PW_(on), PW_(da) and intensity modulation signalPMDATA (that is, a current proportional to the current I_(DA1)), thecurrent directly being a forward current of the semiconductor laser 1.

Generally, the differential quantum efficiency of the semiconductorlaser 1 and the light to electricity conversion sensitivity of the lightreception device 2 vary depending on particular products. Therefore, itis necessary to set a current value in consideration with thesecharacteristics. In consideration with such variation of thesecharacteristics, an external current setting signal is used to set thecurrent I_(DA1) so that a desired light output is emitted from thesemiconductor laser 1. Thus, it is possible set the current so thatcharacteristic variation depending on particular products is compensatedand the semiconductor laser 1 always emits a desired light output in asteady state. When the constant-current source 5 has the arrangementshown in FIG. 7A, 7B or 7C, I_(full) is set so that a desired maximumlight emission can be obtained. The differential quantum efficiency andoscillation threshold current vary greatly depending on the time thesemiconductor laser 1 has been used and temperature. By detecting eachvalue for a condition of use, and driving the semiconductor laser 1 by aforward current so that a desired light intensity is provided, a lightoutput waveform shown in FIG. 2B can be obtained.

The constant-current source 8 may have the arrangement shown in FIG. 7A,7B or 7C. In this case, the maximum current value of the digital toanalog converter will be referred to as I_(full2) ·I_(full2) should beset. How to set I_(full2) will now be described in outline. A forwardcurrent of the semiconductor laser 1 when a desired maximum lightemission is provided will be referred to as I_(max), and a forwardcurrent of the semiconductor laser 1 when a offset light emission isprovided will be referred to as I_(min). As described above, for thepurpose of causing the negative feedback loop 3 to always operate, thesemiconductor laser 1 should not be completely turned off and it isnecessary for the semiconductor laser 1 to slightly emit light. Such acondition of light emission where the semiconductor laser 1 slightlyemits light is referred to as offset light emission. Then, thedifference I_(max) -I_(min) should be equal to I_(full2). For thispurpose, first, the semiconductor laser 1 provides the maximum lightemission where I_(DA2) =0 (that is, I_(full2) =0). At that time, onlythe control current is used to provide a forward current of thesemiconductor laser 1. Therefore, I_(max) =I_(DA1). Then, with themaximum light emission condition maintained, I_(DA2) is graduallyincrease (the light emission instruction signal is ON and I_(full2) iscaused to increase). Then, when I_(DA1) =I_(min), I_(full2) =I_(max)-I_(min). Such a setting operation is performed for a predetermined timeas an initialization operation when the power supply is started or areset operation is performed. Then, during a normal operation condition,the value of I_(full2) is held.

FIG. 51 shows a ninth embodiment of the present invention. In theembodiment, a plurality of data converting units, pulse width modulationunits and light emission instruction signal generating units, which formpulse width modulation and intensity modulation signal generating units,are provided, the number of which is the number of available differentwriting clock frequency output modes. Specifically, a pulse widthmodulation unit 117a and a data converting unit 116a, to which an inputclock signal `a` and input data `a` are input, and a light emissioninstruction signal generating unit 112a are provided. Similarly, a pulsewidth modulation unit 117b and a data converting unit 116b, to which aninput clock signal `b` and input data `b` are input, and a lightemission instruction signal generating unit 112b are provided. n sets ofsuch arrangements are provided. To the output sides of the lightemission instruction signal generating units 112a, 112b, . . . , aselector 881, which selects one of the outputs thereof in accordancewith the frequency selecting signal, and outputs the selected output, isconnected. The selector acts as output mode selecting means. The inputclock signals `a`, `b`, . . . set frequencies of the outputs,respectively, independently.

Thus, by selecting the output of one of the light emission instructionsignal generating units, a light output waveform of a different writingclock frequency can be obtained. For example, the pulse width modulationunit 117a and data converting unit 116a are configured in accordancewith the above-described basic arrangements of the corresponding units,and the output of the relevant light emission instruction signalgenerating unit 112a is selected. Thereby, a multiple tone high qualityimage can be obtained. Further, the input clock signal `b` has afrequency which is double the frequency of the input clock signal `a`,the pulse width modulation unit 117b and data converting unit 116b areconfigured for reducing the number of tone levels, and the output of therelevant light emission instruction signal generating unit 112b isselected. Thereby, a high quality image, in which the number of tonelevels is relatively small but writing density is double, can beobtained. Other than this, by setting a frequency of an input clocksignal and a pulse width modulation unit and configuring a dataconverting unit appropriately, any light output waveform can beobtained.

FIG. 52 shows a tenth embodiment of the present invention. In theembodiment, a pulse width modulation and intensity modulation signalgenerating unit is formed of a plurality of modulation units and thesingle light emission instruction signal generating unit 112 which is anoutput unit. The number of the modulation units is the number ofdifferent available clock frequency output modes. Each of the modulationunits generates pulse width modulation data and intensity modulationdata based on input data. Specifically, a pulse width modulation unit117a and a data converting unit 116a, to which an input clock signal `a`and input data `a` are input, and a pulse width modulation unit 117b anda data converting unit 116b, to which an input clock signal `b` andinput data `b` are input, are provided. n sets of such arrangements areprovided. To the output sides of these modulation units, a PWM selector882 and a PM selector are connected. Respective pulse width modulationsignals PW_(on), PW_(da) are input to the PWM selector 882, andrespective intensity modulation signals PMDATA are input to the PMselector 883. The selectors 882 selects the pulse width modulationsignals PW_(on), PW_(da) of one of the pulse width modulation units117a, 117b, . . . , and the selector 883 selects the pulse intensitymodulation signal PMDATA of one of the data converters 116a, 116b, . . ., according to the frequency selecting signal. These selectors 882, 883output the selected signals to the light emission instruction signalgenerating unit 112.

Also in the embodiment, by selecting the output of one of the modulationunits, a light output waveform of a different writing clock frequencycan be obtained. Especially, in comparison to the arrangement shown inFIG. 51, only the single light emission instruction signal generatingunit 112 is used, and the number of components can be effectivelyreduced in this embodiment. Accordingly, this embodiment is advantageouswhen such an arrangement is formed as an integrated circuit or isminiaturized.

In the arrangement shown in FIG. 41, not only the forcible lightemission instruction signal S_(W2) but also the forcible light cessationinstruction signal S_(W1) can be input. However, it is also possible toform a similar arrangement to which only the forcible light emissioninstruction signal S_(W2) can be input. Such an arrangement issufficiently effective for obtaining a detect pulse or the like.

FIGS. 53, 54, 55 show an eleventh embodiment of the present invention.In the embodiment, selection can be performed between the equal mode anddouble mode as described above. The light emission instruction signalgenerating unit 112 has an arrangement such as that shown in FIG. 53.The arrangement shown in FIG. 53 is obtained using the arrangement shownin FIG. 29A as a base. In the arrangement shown in FIG. 53, a firstdigital to analog converter 465a converts intensity modulation dataPMDATA1 into a current. A non-inverted output current I₁ of the digitalto analog converter 465a is input to a first differential switch(current switch) 466a which allows or does not allows the non-invertedoutput current I₁ flowing therethrough in accordance with one pulseP_(W2). An inverted output current I₁ of the digital to analog converter465a is input to a second differential switch (current switch) 467awhich allows or does not allows the inverted output current I₁ flowingtherethrough in accordance with another pulse P_(W1). Similarly, asecond digital to analog converter 465b converts intensity modulationdata PMDATA2 into a current. A non-inverted output current I₂ of thedigital to analog converter 465b is input to a first differential switch(current switch) 466b which allows or does not allows the non-invertedoutput current I₂ flowing therethrough in accordance with one pulseP_(W4). An inverted output current I₂ of the digital to analog converter465b is input to a second differential switch (current switch) 467bwhich allows or does not allows the inverted output current I₂ flowingtherethrough in accordance with another pulse P_(W3). Thus, the lightemission instruction signal generating unit 112 includes the two digitalto analog converts 465a, 465b and the four differential switches 466a,466b, 467a, 467b. The total current I of the output currents of the fourdifferential switches 466a, 466b, 467a, 467b is a light emissioninstruction signal. There, I₁ =I_(full) -I₁, I₂ =I_(full) -I₂, and eachof the digital to analog converters 465a, 465b is set so that themaximum current value is I_(full).

The pulse width modulation unit 117 which outputs the respective pulsesP_(W1), P_(W2), P_(W3) and P_(W4) includes, for example, as shown inFIG. 54, four multiplexers (selectors) 981, 982, 983, 984, and four ANDgates 985, 986, 987, 988 to which the outputs of the multiplexers 981,982, 983, 984 and the internal clock signal X₀ and the inverted internalclock signal X₀ are input as shown in the figure.

In the above-described arrangement, an example of operation control inthe double mode case will now be described with reference to time chartsof FIG. 55. In the embodiment, during the first half of one input clockperiod, one dot of the light emission instruction signal is generatedusing the differential switches 466a, 467a and the digital to analogconverter 465a. The differential switches 466a, 467a are controlled bythe pulses P_(W1), P_(W2) which are obtained from the AND gates 985,987. The intensity modulation data PMDATA1 is input to the digital toanalog converter 465a. During the second half of one input clock period,one dot of the light emission instruction signal is generated using thedifferential switches 466b, 467b and the digital to analog converter465b. The differential switches 466b, 467b are controlled by the pulsesP_(W3), P_(W4) which are obtained from the AND gates 986, 988. Theintensity modulation data PMDATA2 is input to the digital to analogconverter 465b. Thus, two digital to analog converters 465a, 465b areused, and each of the intensity modulation data PMDATA1, PMDATA2 is usedfor a respective one of the two different dots. Even in the double modecase, the intensity modulation data PMDATA1, PMDATA2 is changed in aperiod the same as the period of the input clock signal, as shown inFIG. 55. Thus, the intensity modulation data PMDATA1, PMDATA2 is changedin a period the same as a period in which the intensity modulation datais changed in the equal mode. Thereby, high-speed data processing can beachieved.

In the above-described seventh, eighth, ninth, tenth and eleventhembodiments, the arrangement is formed as the integrated circuit 13 inone chip, and each part thereof is formed as an integrated circuit bybipolar transistors. However, it is not necessary to form them as anintegrate circuit. When the arrangement is formed as an integratedcircuit, it is not necessary to use bipolar transistors.

The present invention is not limited to the above-described embodiments,and variations and modifications may be made without departing from thescope of the claims.

Any features of the above-described first, second, third, fourth, fifth,sixth, seventh, eighth, ninth and eleventh embodiments may be combinedin any manner as long as they do not contradict each other.Specifically, for example, features of the pulse width generating unitand data modulation unit 11 and features of the semiconductor lasercontrol unit and semiconductor laser driving unit 12 can be combined. Inother words, for particular applications, features of respectiveembodiments may be appropriately used together and thus another suitableembodiment may be obtained.

What is claimed is:
 1. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as a forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit.
 2. Thesemiconductor laser control system according to claim 1, wherein saidpulse width modulation and intensity modulation signal generating unitcomprises:data converting means for converting the input data into pulsemodulation data and intensity modulation data; pulse width modulationmeans which, based on the pulse modulation data, generates apulse-modulated plurality of pulses; and a light emission instructionsignal generating unit which, based on the outputs of said dataconverting means and said pulse width modulation means, performs thepulse width modulation and intensity modulation and generates the lightemission instruction signal for said semiconductor laser.
 3. Thesemiconductor laser control system according to claim 1, wherein saidone chip of the integrated circuit is formed by bipolar transistors. 4.The semiconductor laser control system according to claim 1, whereinsaid one chip of the integrated circuit is formed by C-MOS transistors.5. The semiconductor laser control system according to claim 1, whereinsaid one chip of the integrated circuit is formed of by a composite ofbipolar transistors and C-MOS transistors.
 6. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation generating unit which, based on input data, performs pulsewidth modulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as a forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and an emitter coupled logic circuit included in an inputportion of said pulse width modulation and intensity modulation signalgenerating unit into which the input data is input.
 7. A semiconductorlaser control system, comprising:a pulse width modulation and intensitymodulation generating unit which, based on input data, performs pulsewidth modulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and an input portion of said pulse width modulation andintensity modulation signal generating unit into which the input data isinput is an impedance matching circuit.
 8. A semiconductor laser controlsystem, comprising:a pulse width modulation and intensity modulationgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and the input data is input to said pulse width modulationand intensity modulation signal generating unit in a combination of apositive logic signal and a negative logic signal, in parallel throughtwo lines.
 9. A semiconductor laser control system, comprising:a pulsewidth modulation and intensity modulation generating unit which, basedon input data, performs pulse width modulation and intensity modulationand generates a light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, wherein:said pulsewidth modulation and intensity modulation signal generating unit, saiderror amplifier and said current driving unit are formed as one chip ofan integrated circuit by bipolar transistors; and an emitter coupledlogic circuit is included in an input portion of said pulse widthmodulation and intensity modulation signal generating unit to receivethe input data, and an input portion of said emitter coupled logiccircuit is an impedance matching circuit.
 10. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation generating unit which, based on input data, performs pulsewidth modulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and an input portion of said pulse width modulation andintensity modulation signal generating unit into which the input data isinput is an impedance matching circuit, and the input data is input tosaid pulse width modulation and intensity modulation signal generatingunit in a combination of a positive logic signal and a negative logicsignal, in parallel through two lines.
 11. A semiconductor laser controlsystem, comprising:a pulse width modulation and intensity modulationgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and an emitter coupled logic circuit included in an inputportion of said pulse width modulation and intensity modulation signalgenerating unit to receive the input data, an input portion of saidemitter coupled logic circuit is an impedance matching circuit, and theinput data is input to said emitter coupled logic circuit in acombination of a positive logic signal and a negative logic signal, inparallel through two lines.
 12. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current to flow throughsaid semiconductor laser as the forward current, the driving currentbeing generated so as to control driving of said semiconductor laserwith a current of one of a difference and sum with the control currentof said negative feedback loop, wherein:said pulse width modulation andintensity modulation signal generating unit, said error amplifier andsaid current driving unit are formed as one chip of an integratedcircuit by bipolar transistors; and said pulse width modulation andintensity modulation signal generating unit comprises a light emissioninstruction signal generating unit which performs pulse width modulationand intensity modulation and generates the light emission instructionsignal for said semiconductor laser, an external connection device beingprovided for setting a current value of said light emission instructionsignal generating unit.
 13. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit by bipolartransistors; and an external connection device is provided for setting acontrol speed of said negative feedback loop.
 14. A semiconductor lasercontrol system comprising:a pulse width modulation and intensitymodulation signal generating unit which, based on input data, performspulse width modulation and intensity modulation and generates a lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; and a current driving unit providing a driving current,according to the light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop, a differential quantum efficiency detecting unitfor detecting the differential quantum efficiency of said semiconductorlaser; a memory unit for storing a detection result of said differentialquantum efficiency detecting unit; an adding current setting unit forsetting a current corresponding to the light emission instructionsignal, using the detection result stored in said memory unit; and atiming generating unit, wherein, in initialization, said timinggenerating unit generates a timing signal slower than a control speed ofsaid error amplifier, said differential quantum efficiency detectingunit detects the differential quantum efficiency of said semiconductorlaser based on said timing signal, said memory unit stores a detectionresult at each timing, and the current corresponding to the lightemission instruction signal is set using the stored detection results,and wherein said pulse width modulation and intensity modulation signalgenerating unit, said error amplifier, said current driving unit, saiddifferential quantum efficiency detecting unit, said memory unit, saidadding current setting unit and said timing generating unit are formedas one chip of an integrated circuit.
 15. The semiconductor lasercontrol system according to claim 14, wherein said timing generatingunit includes an external connection device, said timing generating unitgenerating the timing signal set by said external connection device. 16.The semiconductor laser control system according to claim 14, whereinsaid timing generating unit includes an oscillation circuit, said timinggenerating unit generating a plurality of timing signals based on anoscillation output of said oscillation circuit.
 17. The semiconductorlaser control system according to claim 14, wherein said timinggenerating unit includes an oscillation circuit and multiple stages oflatch circuits, each latch circuit generating the timing signal based onan oscillation output of said oscillation circuit.
 18. A semiconductorlaser control system comprising:a pulse width modulation and intensitymodulation signal generating unit which, based on input data, performspulse width modulation and intensity modulation and generates a lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; and a current driving unit providing a driving current,according to the light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop, a differential quantum efficiency detecting unitfor detecting the differential quantum efficiency of said semiconductorlaser; a timing generating unit for generating a timing signal whichcontrols a detection operation of said differential quantum efficiencydetecting unit in initialization; a memory unit for storing a detectionresult of said differential quantum efficiency detecting unit at eachtiming; and an adding current setting unit for setting a current,corresponding to the light emission instruction signal, using thedetection results stored by said memory unit, wherein said pulse widthmodulation and intensity modulation signal generating unit, said erroramplifier, said current driving unit, said differential quantumefficiency detecting unit, said timing generating unit, said memoryunit, and said adding current setting unit are formed as one chip of anintegrated circuit.
 19. The semiconductor laser control system accordingto claim 18, wherein:said current driving unit comprises a voltageshifting unit in said error amplifier, includes a differential circuitfor changing an amount of voltage shift, and is provided in saidnegative feedback loop; said adding current setting unit sets a currentof said differential circuit so that light output of said semiconductorlaser is a maximum value when a current corresponding to the lightemission instruction signal is maximum, and light output of saidsemiconductor laser is a minimum value when a current corresponding tothe light emission instruction signal is minimum; in initialization,light output of said semiconductor laser is set to said maximum value ata certain timing T0, light output of said semiconductor laser is set tosaid minimum value at a timing T1 after a fixed time has elapsed fromsaid timing T0, said differential quantum efficiency detecting unit andsaid adding current setting unit operate and the current is set betweensaid timing T1 and a timing T2 after a fixed time has elapsed from saidtiming T1.
 20. The semiconductor laser control system according to claim18, wherein said timing generating circuit includes an externalconnection device, said timing generating unit generating the timingsignal set by said external connection device.
 21. The semiconductorlaser control system according to claim 18, wherein said timinggenerating unit includes an oscillation circuit which generates aplurality of timing signals based on an oscillation output.
 22. Thesemiconductor laser control system according to claim 18, wherein saidtiming generating circuit includes an oscillation circuit and multiplestages of latch circuits which generate respective timing signals basedon an oscillation output of said oscillation circuit.
 23. Asemiconductor laser control system, comprising:a pulse width modulationand intensity modulation signal generating unit which, based on inputdata, performs pulse width modulation and intensity modulation andgenerates a light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, a differentialquantum efficiency detecting unit for detecting the differential quantumefficiency of said semiconductor laser; a memory unit for storing adetection result of said differential quantum efficiency detecting unit;an adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection result storedin said memory unit; and a timing generating unit, wherein:said pulsewidth modulation and intensity modulation signal generating unit, saiderror amplifier, said current driving unit, said differential quantumefficiency detecting unit, said memory unit, said adding current settingunit and said timing generating unit are formed as one chip of anintegrated circuit by bipolar transistors; and in initialization, saidtiming generating unit generates a timing signal slower than a controlspeed of said error amplifier, said differential quantum efficiencydetecting unit detects the differential quantum efficiency of saidsemiconductor laser based on the timing signal, said memory units storesa detection result at each timing, and the current corresponding to thelight emission instruction signal is set using the stored detectionresults.
 24. The semiconductor laser control system according to claim23, wherein said timing generating unit includes an external connectiondevice, said timing generating unit generating the timing signal set bysaid external connection device.
 25. The semiconductor laser controlsystem according to claim 23, wherein said timing generating unitincludes an oscillation circuit, said timing generating unit generatinga plurality of timing signals based on an oscillation output of saidoscillation circuit.
 26. The semiconductor laser control systemaccording to claim 23, wherein said timing generating unit includes anoscillation circuit and multiple stages of latch circuits, each latchcircuit generating the timing signal based on an oscillation output ofsaid oscillation circuit.
 27. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal; to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of said semiconductor laser; a timinggenerating unit for generating a timing signal which controls adetection operation of said differential quantum efficiency detectingunit in initialization; a memory unit for storing a detection result ofsaid differential quantum efficiency detecting unit at each timing; andan adding current setting unit for setting a current, corresponding tothe light emission instruction signal, using the detection resultsstored by said memory unit, wherein said pulse width modulation andintensity modulation signal generating unit, said error amplifier, saidcurrent driving unit, said differential quantum efficiency detectingunit, said timing generating unit, said memory unit, and said addingcurrent setting unit are formed as one chip of an integrated circuit bybipolar transistors.
 28. The semiconductor laser control systemaccording to claim 27, wherein:said current driving unit comprises avoltage shifting unit in said error amplifier, includes a differentialcircuit for changing an amount of voltage shift, and is provided in saidnegative feedback loop; said adding current setting unit sets a currentof said differential circuit so that light output of said semiconductorlaser is a maximum value when a current corresponding to the lightemission instruction signal is maximum, and light output of saidsemiconductor laser is a minimum value when a current corresponding tothe light emission instruction signal is minimum; in initialization,light output of said semiconductor laser is set to said maximum value ata certain timing T0, light output of said semiconductor laser is set tosaid minimum value at a timing T1 after a fixed time has elapsed fromsaid timing T0, said differential quantum efficiency detecting unit andsaid adding current setting unit operate and the current is set betweensaid timing T1 and a timing T2 after a fixed time has elapsed from saidtiming T1.
 29. The semiconductor laser control system according to claim27, wherein said timing generating unit includes an external connectiondevice, said timing generating unit generating the timing signal set bysaid external connection device.
 30. The semiconductor laser controlsystem according to claim 27, wherein said timing generating unitincludes an oscillation circuit which generates a plurality of timingsignals based on an oscillation output.
 31. The semiconductor lasercontrol system according to claim 27, wherein said timing generatingunit includes an oscillation circuit and multiple stages of latchcircuits which generates the timing signal based on an oscillationoutput of said oscillation circuit.
 32. A semiconductor laser controlsystem, comprising:a pulse width modulation and intensity modulationsignal generating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of said semiconductor laser; a memoryunit for storing a detection result of said differential quantumefficiency detecting unit; an adding current setting unit for setting acurrent, corresponding to the light emission instruction signal, usingthe detection result stored in said memory unit; a timing generatingunit; and a switch unit, to which one of a forcible light emissioninstruction signal and a forcible light cessation instruction signal isselectively input, said switch unit providing an output selected fromoutputs including the light emission instruction signal based on inputdata; wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier, said current driving unit,said differential quantum efficiency detecting unit, said memory unit,said adding current setting unit and said timing generating unit areformed as one chip of an integrated circuit by bipolar transistors; andin initialization, said timing generating unit generates a timing signalslower than a control speed of said error amplifier, said differentialquantum efficiency detecting unit detects the differential quantumefficiency of said semiconductor laser based on the timing signal, saidmemory unit stores a detection result at each timing, and the currentcorresponding to one of the light emission instruction signal and theforcible light emission instruction signal is set using the storeddetection results.
 33. A semiconductor laser control system comprising:apulse modulation and intensity modulation signal generating unit which,based on input data, performs pulse width modulation and intensitymodulation and generates a light emission instruction signal; an erroramplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, a switch unit, to which a forcible light emission instructionsignal and a forcible light cessation instruction signal are selectivelyinput, said switch unit providing an output selected from outputsincluding the light emission instruction signal based on input data; adifferential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of said semiconductor laser based on atiming signal; a timing generating unit for generating a timing signalwhich is slower than a control speed of said error amplifier, forcontrolling a detection operation of said differential quantumefficiency detecting unit, in initialization; a memory unit for storinga detection result of said differential quantum efficiency detectingunit at each timing; and an adding current setting unit for setting acurrent, corresponding to one of the light emission instruction signaland the forcible light emission instruction signal, using the detectionresults stored by said memory unit; wherein said pulse width modulationand intensity modulation signal generating unit, said error amplifier,said current driving unit, said switch unit, said differential quantumefficiency detecting unit, said timing generating unit, said memory unitand said adding current setting unit are formed as one chip of anintegrated circuit.
 34. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit; and said currentdriving unit is provided in said negative feedback loop.
 35. Thesemiconductor laser control system according to claim 34, wherein anexternal connection device is provided for setting a current value ofsaid pulse width modulation and intensity modulation signal generatingunit.
 36. The semiconductor laser control system according to claim 34,wherein an external connection device is provided for setting a controlspeed of said negative feedback loop.
 37. The semiconductor lasercontrol system according to claim 34, wherein:said current driving unitcomprises a high-speed voltage shifting unit in said error amplifier,includes a differential circuit for changing an amount of voltage shift,and is provided in said negative feedback loop.
 38. The semiconductorlaser control system according to claim 34, wherein, in said pulse widthmodulation and intensity modulation signal generating unit, a pluralityof digital to analog converting circuit have a common circuit portion.39. The semiconductor laser control system according to claim 34,wherein an offset setting unit is provided which has an externalconnection device for setting an offset current used for setting lightoutput of said semiconductor laser to be a minimum value.
 40. Thesemiconductor laser control system according to claim 34, wherein asimultaneous setting unit is provided having an external connectiondevice for setting, simultaneously, a maximum current of said pulsewidth modulation and intensity modulation signal generating unit and anoffset current which sets light output of said semiconductor laser to aminimum value.
 41. The semiconductor laser control system according toclaim 34, wherein;an offset setting unit is provided which has anexternal connection device for setting an offset current used forsetting a light output of said semiconductor laser to a minimum value;and a simultaneous setting unit is provided having an externalconnection device for setting, simultaneously, a maximum current of saidpulse width modulation and intensity modulation signal generating unitand the offset current.
 42. The semiconductor laser control systemaccording to claim 34, wherein a starting-up unit is provided forallowing operation start when a power source voltage reaches apredetermined voltage in power supply starting.
 43. The semiconductorlaser control system according to claim 34, wherein a light emissioninstruction signal generating unit, providing an absolute currentdetermined by said light reception device, has a base currentcompensation unit for compensating base currents of transistorsconnected to a path of a reference current.
 44. The semiconductor lasercontrol system according to claim 34, wherein:an external connectiondevice is provided for setting a current value of said pulse widthmodulation and intensity modulation signal generating unit; and a lightemission instruction signal generating unit, providing an absolutecurrent determined by said light reception device, has a base currentcompensation unit for compensating base currents of transistorsconnected to a path of a reference current.
 45. The semiconductor lasercontrol system according to claim 34, wherein a plurality of said pulsewidth modulation and intensity modulation signal generating units areprovided.
 46. The semiconductor laser control system according to claim34, wherein an input terminal of an external control voltage forchanging a maximum current of said pulse width modulation and intensitymodulation signal generating unit is provided.
 47. The semiconductorlaser control system according to claim 46, wherein a starting-up unitis provided for allowing operation start when a light emissioninstruction current corresponding to the light emission instructionsignal reaches a predetermined current in power supply starting.
 48. Thesemiconductor laser control system according to claim 46, wherein anadding current setting unit for setting a driving current correspondingto the light emission instruction signal and a simultaneous changingunit for causing a maximum current of said pulse width modulation andintensity modulation signal generating unit and a maximum current ofsaid adding current setting unit to change simultaneously are provided.49. The semiconductor laser control system according to claim 46,wherein a starting-up unit for allowing operation start when a lightemission instruction current corresponding to the light emissioninstruction signal reaches a predetermined current in power supplystarting, an adding current setting unit for setting a driving currentcorresponding to the light emission instruction signal and asimultaneous changing unit for causing a maximum current of said pulsewidth modulation and intensity modulation signal generating unit and amaximum current of said adding current setting unit to changesimultaneously are provided.
 50. The semiconductor laser control systemaccording to claim 46, wherein a first starting-up unit for allowingoperation start when a power source voltage reaches a predeterminedvoltage in power supply starting and a second starting-up unit forallowing operation start when a light emission instruction currentcorresponding to the light emission instruction signal reaches apredetermined current in power supply starting are provided.
 51. Asemiconductor laser control system, comprising:a pulse width modulationand intensity modulation signal generating unit which, based on inputdata, performs pulse width modulation and intensity modulation andgenerates a light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, wherein:said pulsewidth modulation and intensity modulation signal generating unit, saiderror amplifier and said current driving unit are formed as one chip ofan integrated circuit by bipolar transistors; and said current drivingunit is provided in said negative feedback loop.
 52. The semiconductorlaser control system according to claim 51, wherein an externalconnection device is provided for setting a current value of said pulsewidth modulation and intensity modulation signal generating unit. 53.The semiconductor laser control system according to claim 51, wherein anexternal connection device is provided for setting a control speed ofsaid negative feedback loop.
 54. The semiconductor laser control systemaccording to claim 51, wherein:said current driving unit comprises ahigh-speed voltage shifting unit in said error amplifier, includes adifferential circuit for changing an amount of voltage shift, and isprovided in said negative feedback loop.
 55. The semiconductor lasercontrol system according to claim 51, wherein, in said pulse widthmodulation and intensity modulation signal generating unit, a pluralityof digital to analog converting circuit have a common circuit portion.56. The semiconductor laser control system according to claim 51,wherein an offset setting unit is provided which has an externalconnection device for setting an offset current which is used forsetting light output of said semiconductor laser to a minimum value. 57.The semiconductor laser control system according to claim 51, wherein asimultaneous setting unit is provided having an external connectiondevice for setting, simultaneously, a maximum current of said pulsewidth modulation and intensity modulation signal generating unit and anoffset current setting light output of said semiconductor laser to aminimum value.
 58. The semiconductor laser control system according toclaim 51, wherein;an offset setting unit is provided which has anexternal connection device for setting an offset current used forsetting light output of said semiconductor laser to a minimum value; anda setting unit is provided having an external connection device forsetting, simultaneously, a maximum current of said pulse widthmodulation and intensity modulation signal generating unit and theoffset current.
 59. The semiconductor laser control system according toclaim 51, wherein a starting-up unit is provided for allowing operationstart when a power source voltage reaches a predetermined voltage inpower supply starting.
 60. The semiconductor laser control systemaccording to claim 51, wherein a light emission instruction signalgenerating unit, providing an absolute current determined by said lightreception device, has a base current compensation unit for compensatingbase currents of transistors connected to a path of a reference current.61. The semiconductor laser control system according to claim 51,wherein:an external connection device is provided for setting a currentvalue of said pulse width modulation and intensity modulation signalgenerating unit; and a light emission instruction signal generatingunit, providing an absolute current determined by said light receptiondevice, has a base current compensation unit for compensating basecurrents of transistors connected to a path of a reference current. 62.The semiconductor laser control system according to claim 51, wherein aplurality of said pulse width modulation and intensity modulation signalgenerating units are provided.
 63. The semiconductor laser controlsystem according to claim 51, wherein an input terminal of an externalcontrol voltage for changing a maximum current of said pulse widthmodulation and intensity modulation signal generating unit is provided.64. The semiconductor laser control system according to claim 63,wherein a starting-up unit is provided for allowing operation start whena light emission instruction current corresponding to the light emissioninstruction signal reaches a predetermined current in power supplystarting.
 65. The semiconductor laser control system according to claim63, wherein an adding current setting unit for setting a driving currentcorresponding to the light emission instruction signal and asimultaneous changing unit for causing a maximum current of said pulsewidth modulation and intensity modulation signal generating unit and amaximum current of said adding current setting unit to changesimultaneously are provided.
 66. The semiconductor laser control systemaccording to claim 63, wherein a starting-up unit for allowing operationstart when a light emission instruction current corresponding to thelight emission instruction signal reaches a predetermined current inpower supply starting, an adding current setting unit for setting adriving current corresponding to the light emission instruction signaland a simultaneous changing unit for causing a maximum current of saidpulse width modulation and intensity modulation signal generating unitand a maximum current of said adding current setting unit to changesimultaneously are provided.
 67. The semiconductor laser control systemaccording to claim 63, including a first starting-up unit for allowingoperation start when a power source voltage reaches a predeterminedvoltage in power supply starting and a second starting-up unit forallowing operation start when a light emission instruction currentcorresponding to the light emission instruction signal reaches apredetermined current in power supply starting.
 68. A semiconductorlaser control system, comprising:a pulse width modulation and intensitymodulation signal generating unit which comprises data converting meansfor converting input data into pulse modulation data and intensitymodulation data, pulse width modulation means which, based on the pulsemodulation data, generates a plurality of pulse-modulated pulses, and alight emission instruction signal generating unit which, based onoutputs of said data converting means and said pulse width modulationmeans, performs the pulse width modulation and intensity modulation andgenerates a light emission instruction signal for said semiconductorlaser, said pulse width modulation and intensity modulation signalgenerating unit, based on input data, performing pulse width modulationand intensity modulation so as to generate the light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal;and a current driving unit providing a driving current, according to thelight emission instruction signal, to flow through said semiconductorlaser as the forward current, the driving current being generated so asto control driving of said semiconductor laser with a current of one ofa difference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit: and an externalconnection device provided for setting a current value of said lightemission instruction signal generating unit.
 69. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation signal generating unit which, based on input data, performspulse width modulation and intensity modulation and generates a lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; and a current driving unit providing a driving current,according to the light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop, wherein:said pulse width modulation andintensity modulation signal generating unit, said error amplifier andsaid current driving unit are formed as one chip of an integratedcircuit: and an external connection device is provided for setting acontrol speed of said negative feedback loop.
 70. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation signal generating unit which, based on input data, performspulse width modulation and intensity modulation and generates a lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; a current driving unit providing a driving current, according tothe light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop; and a starting-up unit for allowing operationstart when a power source voltage reaches a predetermined voltage inpower supply starting, wherein said pulse width modulation and intensitymodulation signal generating unit, said error amplifier, said currentdriving unit and said starting-up unit are formed as one chip of anintegrated circuit.
 71. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal; acurrent driving unit providing a driving current, according to the lightemission instruction signal, to flow through said semiconductor laser asthe forward current , the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; and a starting-up unit for allowing operation start when a powersource voltage reaches a predetermined voltage in power supply starting,wherein:said pulse width modulation and intensity modulation signalgenerating unit, said error amplifier, said current driving unit andsaid starting-up unit are formed as one chip of an integrated circuit;and said starting-up unit has a reset function for initializing saidintegrated circuit by a control signal external of said integratedcircuit.
 72. A semiconductor laser control system, comprising:a pulsewidth modulation and intensity modulation signal generating unitcomprising a plurality of light emission instruction signal generatingunits which, based on input data, perform pulse width modulation andintensity modulation and generate a light emission instruction signal;an error amplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; wherein said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit.
 73. Asemiconductor laser control system, comprising:a pulse width modulationand intensity modulation signal generating unit which comprises dataconverting means for converting input data into pulse modulation dataand intensity modulation data, pulse width modulation means which, basedon the pulse modulation data, generates a plurality of pulse-modulatedpulses, and a light emission instruction signal generating unit which,based on the outputs of said data converting means and said pulse widthmodulation means, performs the pulse width modulation and intensitymodulation and generates a light emission instruction signal for saidsemiconductor laser, said pulse width modulation and intensitymodulation signal generating unit, based on input data, performing pulsewidth modulation and intensity modulation and generates the lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; and a current driving unit providing a driving current,according to the light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop, wherein:said pulse width modulation andintensity modulation signal generating unit, said error amplifier andsaid current driving unit are formed as one chip of an integratedcircuit: and an input terminal is provided for an external controlvoltage which changes a maximum current of said light emissioninstruction signal generating unit.
 74. The semiconductor laser controlsystem according to claim 73, wherein a starting-up unit is provided forallowing operation start when a light emission instruction currentcorresponding to the light emission instruction signal reaches apredetermined current in power supply starting.
 75. The semiconductorlaser control system according to claim 73, wherein an adding currentsetting unit for setting a driving current corresponding to the lightemission instruction signal and a simultaneous changing unit for causinga maximum current of said light emission instruction signal generatingunit and a maximum current of said adding current setting unit to changesimultaneously are provided.
 76. The semiconductor laser control systemaccording to claim 73, wherein a starting-up unit for allowing operationstart when a light emission instruction current corresponding to thelight emission instruction signal reaches a predetermined current inpower supply starting, an adding current setting unit for setting adriving current corresponding to the light emission instruction signaland a simultaneous changing unit for causing a maximum current of saidpulse width modulation and intensity modulation signal generating unitand a maximum current of said adding current setting unit to changesimultaneously are provided.
 77. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal; acurrent driving unit providing a driving current, according to the lightemission instruction signal, to flow through said semiconductor laser asthe forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; and an offset setting unit having an external connection devicefor setting an offset current which sets light output of saidsemiconductor laser to a minimum value, wherein said pulse widthmodulation and intensity modulation signal generating unit, said erroramplifier, said current driving unit and said offset setting unit areformed as one chip of an integrated circuit.
 78. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation signal generating unit which comprises data converting meansfor converting input data into pulse modulation data and intensitymodulation data, pulse width modulation means which, based on the pulsemodulation data, generates a plurality of pulse-modulated pulses, and alight emission instruction signal generating unit which, based on theoutputs of said data converting means and said pulse width modulationmeans, performs the pulse width modulation and intensity modulation andgenerates a light emission instruction signal for said semiconductorlaser, said pulse width modulation and intensity modulation signalgenerating unit, based on input data, performing pulse width modulationand intensity modulation and generates the light emission instructionsignal; an error amplifier providing a negative feedback loop togetherwith a semiconductor laser and a light reception device which monitorslight output of said semiconductor laser, said error amplifiercontrolling forward current of said semiconductor laser so that a lightreception signal proportional to the light output of said semiconductorlaser is equal to the light emission instruction signal; a currentdriving unit providing a driving current, according to the lightemission instruction signal, to flow through said semiconductor laser asthe forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; and a simultaneous setting unit having an external connectiondevice for setting, simultaneously, a maximum current of said lightemission instruction signal generating unit and an offset current whichcauses light output of said semiconductor laser to be a minimum value,wherein said pulse width modulation and intensity modulation signalgenerating unit, said error amplifier, said current driving unit andsaid together setting unit are formed as one chip of an integratedcircuit.
 79. A semiconductor laser control system, comprising:a pulsewidth modulation and intensity modulation signal generating unit whichcomprises data converting means for converting input data into pulsemodulation data and intensity modulation data, pulse width modulationmeans which, based on the pulse modulation data, generates a pluralityof pulse-modulated pulses, and a light emission instruction signalgenerating unit which, based on the outputs of said data convertingmeans and said pulse width modulation means, performs the pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal for said semiconductor laser, said pulse widthmodulation and intensity modulation signal generating unit, based oninput data, performing pulse width modulation and intensity modulationand generates the light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; a current driving unit providing a drivingcurrent, according to the light emission instruction signal, to flowthrough said semiconductor laser as the forward current, the drivingcurrent being generated so as to control driving of said semiconductorlaser with a current of one of a difference and sum with the controlcurrent of said negative feedback loop; an offset setting unit having anexternal connection device for setting an offset current which setslight output of said semiconductor laser to a minimum value; and asimultaneous setting unit having an external connection device forsetting, simultaneously, a maximum current of said light emissioninstruction signal generating unit and the offset current, wherein saidpulse width modulation and intensity modulation signal generating unit,said error amplifier, said current driving unit, said offset settingunit and said together setting unit are formed as one chip of anintegrated circuit.
 80. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which, based on input data, performs pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal; acurrent driving unit providing a driving current, according to the lightemission instruction signal, to flow through said semiconductor laser asthe forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; and an abnormality detecting unit for detecting an abnormality ina signal from an output terminal of said semiconductor laser, whereinsaid pulse width modulation and intensity modulation signal generatingunit, said error amplifier, said current driving unit and saidabnormality detecting unit are formed as one chip of an integratedcircuit.
 81. A semiconductor laser control system, comprising:a pulsewidth modulation and intensity modulation signal generating unit whichcomprises data converting means for converting input data into pulsemodulation data and intensity modulation data, pulse width modulationmeans which, based on the pulse modulation data, generates a pluralityof pulse-modulated pulses, and a light emission instruction signalgenerating unit which, based on outputs of said data converting meansand said pulse width modulation means, performs the pulse widthmodulation and intensity modulation and generates a light emissioninstruction signal for said semiconductor laser, said pulse widthmodulation and intensity modulation signal generating unit, based oninput data, performing pulse width modulation and intensity modulationand generates the light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, wherein:said pulsewidth modulation and intensity modulation signal generating unit, saiderror amplifier and said current driving unit are formed as one chip ofan integrated circuit; and said light emission instruction signalgenerating unit comprises a base current compensation unit forcompensating base currents of transistors connected to a path of areference current.
 82. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit which comprises data converting means for convertinginput data into pulse modulation data and intensity modulation data,pulse width modulation means which, based on the pulse modulation data,generates a plurality of pulse-modulated pulses, and a light emissioninstruction signal generating unit which, based on the outputs of saiddata converting means and said pulse width modulation means, performsthe pulse width modulation and intensity modulation and generates alight emission instruction signal for said semiconductor laser, saidpulse width modulation and intensity modulation signal generating unit,based on input data, performing pulse width modulation and intensitymodulation and generates the light emission instruction signal; an erroramplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said,semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein:said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit; an externalconnection device is provided for setting a current value of said lightemission instruction signal generating unit; and said light emissioninstruction signal generating unit, providing an absolute currentdetermined by said light reception device, has a base currentcompensation unit for compensating base currents of transistorsconnected to a path of a reference current.
 83. A semiconductor lasercontrol system, comprising:a pulse width modulation and intensitymodulation signal generating unit which, based on input data, performspulse width modulation and intensity modulation and generates a lightemission instruction signal; an error amplifier providing a negativefeedback loop together with a semiconductor laser and a light receptiondevice which monitors light output of said semiconductor laser, saiderror amplifier controlling forward current of said semiconductor laserso that a light reception signal proportional to the light output ofsaid semiconductor laser is equal to the light emission instructionsignal; a current driving unit providing a driving current, according tothe light emission instruction signal, to flow through saidsemiconductor laser as the forward current, the driving current beinggenerated so as to control driving of said semiconductor laser with acurrent of one of a difference and sum with the control current of saidnegative feedback loop; and output mode change-over means for selectingone of different clock-frequency output modes according to a frequencyselecting signal, wherein said pulse width modulation and intensitymodulation signal generating unit, said error amplifier, said currentdriving unit and said output mode change-over means are formed as onechip of an integrated circuit.
 84. The semiconductor laser controlsystem according to claim 83, wherein:a plurality of said pulsemodulation and intensity modulation signal generating units areprovided, the number of said plurality of said pulse modulation andintensity modulation signal generating units being equal to the numberof said different clock-frequency output modes; and said output modechange-over means selecting one of said plurality of said pulsemodulation and intensity modulation signal generating units according tothe frequency selecting signal.
 85. The semiconductor laser controlsystem according to claim 83, wherein:said pulse width modulation andintensity modulation signal generating unit comprises a plurality ofmodulation units for generating pulse width modulation data andintensity modulation data from input data, the number of said pluralityof said modulation units being equal to the number of said differentclock-frequency output modes, and one output unit which performs pulsewidth modulation and intensity modulation based on the pulse modulationdata and intensity modulation data, and generates the light emissioninstruction signal; and said output mode change-over means selecting oneof said plurality of modulation units for said output unit according tothe frequency selecting signal.
 86. The semiconductor laser controlsystem according to claim 83, wherein said pulse width modulation andintensity modulation signal generating unit comprises pulse generatingmeans for generating a plurality of pulses having a frequency equal to afrequency of an input clock signal and having different phases, thephase difference being a fixed phase difference, data converting meansfor converting input data into pulse width modulation data and intensitymodulation data and pulse width modulation means for generating aplurality of pulses, which have undergone pulse width modulation basedon the pulse width modulation data, from the pulses generated by saidpulse generating means; andsaid output mode change-over means causessaid data converting means to generate modulation data according to thefrequency selecting signal and output said modulation data to said pulsewidth modulation means.
 87. The semiconductor laser control systemaccording to claim 86, wherein the clock frequencies of the output modescomprise a first clock frequency which is equal to the clock frequencyof the input clock signal and a second clock frequency which is doublethe clock frequency of the input clock signal, one of said first clockfrequency and said second clock frequency being selectable.
 88. Thesemiconductor laser control system according to claim 83, wherein one ofsaid different clock-frequency output modes is an output mode selectedbased on a forcible light emission instruction signal which comprisesthe frequency selecting signal.
 89. The semiconductor laser controlsystem according to claim 83, wherein one of said differentclock-frequency output modes is an output mode selected based on aforcible light cessation signal which comprises the frequency selectingsignal.
 90. A semiconductor laser control system, comprising:a pulsewidth modulation and intensity modulation signal generating unitcomprising pulse generating means, including a pulse oscillator, forgenerating a plurality of pulses having a frequency the same as thefrequency of an input clock signal and having different phases, thephase difference being a fixed phase difference, data converting meansfor converting input image data into pulse width modulation data andintensity modulation data and pulse width modulation means forgenerating a plurality of pulses, which have undergone pulse widthmodulation based on the pulse width modulation data, from the pulsesgenerated by said pulse generating means, said pulse width modulationand intensity modulation signal generating unit performing pulse widthmodulation and intensity modulation and generating a light emissioninstruction signal; an error amplifier providing a negative feedbackloop together with a semiconductor laser and a light reception devicewhich monitors light output of said semiconductor laser, said erroramplifier controlling forward current of said semiconductor laser sothat a light reception signal proportional to the light output of saidsemiconductor laser is equal to the light emission instruction signal; acurrent driving unit providing a driving current, according to the lightemission instruction signal, to flow through said semiconductor laser asthe forward current, the driving current being generated so as tocontrol driving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop; and wherein said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as,one chip of an integrated circuit.
 91. Thesemiconductor laser control system according to claim 90, wherein saidpulse width modulation means comprises n sets of two selectors forselecting pulses, based on mutually different pulse width modulationdata, from pulses generated by said pulse generating means, two logicAND gates to which the outputs of said selectors and a non-invertedinternal clock signal and an inverted internal clock signal are input,and a logic OR gate, to which the outputs of said logic AND gates areinput, for outputting pulses.
 92. The semiconductor laser control systemaccording to claim 91, wherein clock frequencies of output modescomprise a first clock frequency which is equal to an input clockfrequency and a second clock frequency which is double said input clockfrequency, one of said first clock frequency and said second clockfrequency being selectable.
 93. The semiconductor laser control systemaccording to claim 90, wherein said data converting means, based on theimage data and a frequency selecting signal, converts the image datainto pulse width modulation data and intensity modulation data.
 94. Thesemiconductor laser control system according to claim 90, wherein saiddata converting means, based on the image data, a frequency selectingsignal and a dot position control signal, converts the image data intopulse width modulation data and intensity modulation data.
 95. Thesemiconductor laser control system according to claim 94, wherein saiddot position control signal comprises one bit.
 96. The semiconductorlaser control system according to claim 90, wherein means for generatingthe light emission instruction signal comprises a digital to analogconverter, a first current switch which controls a non-inverted outputcurrent of said digital to analog converter flowing therethroughaccording to one of the pulses output from said pulse width modulationmeans and a second current switch which controls an inverted outputcurrent of said digital to analog converter flowing therethroughaccording to another of said pulses output from said pulse widthmodulation means, total output currents of said first and second currentswitches being used as the light emission instruction signal.
 97. Thesemiconductor laser control system according to claim 90, wherein meansfor generating the light emission instruction signal comprises a digitalto analog converter, a first current switch which controls anon-inverted output current of said digital to analog converter flowingtherethrough according to one of the pulses output from said pulse widthmodulation means and a second current switch which controls an invertedoutput current of said digital to analog converter and a constantcurrent equal to the least significant bit current of said digital toanalog converter flowing therethrough according to another of saidpulses output from said pulse width modulation means, total outputcurrents of said first and second current switches being used as thelight emission instruction signal.
 98. The semiconductor laser controlsystem according to claim 97, wherein a third current switch is providedfor controlling a constant current flowing therethrough based on a fullon signal.
 99. The semiconductor laser control system according to claim90, wherein:output mode clock frequencies comprise a first clockfrequency which is equal to the input clock signal and a second clockfrequency which is double said input clock signal, one of said firstclock frequency and said second clock frequency being selectable; andmeans for generating the light emission instruction signal comprisingtwo digital to analog converters which convert two intensity modulationdata into currents, two first current switches which controlnon-inverted output currents of said two digital to analog convertersflowing therethrough according to respective ones of the pulses outputfrom said pulse width modulation means and two second current switcheswhich control inverted output currents of said two digital to analogconverters flowing therethrough according to other respective ones ofsaid pulses output from said pulse width modulation means, total outputcurrents of the current switches being used as the light emissioninstruction signal.
 100. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit comprising pulse generating means for generating aplurality of pulses having a frequency equal to a frequency of an inputclock signal and having different phases, the phase difference being afixed phase difference, data converting means for converting input imagedata into pulse width modulation data and intensity modulation data andpulse width modulation means for generating a plurality of pulses, whichhave undergone pulse width modulation based on the pulse widthmodulation data, from the pulses generated by said pulse generatingmeans, said pulse width modulation and intensity modulation signalgenerating unit performing pulse width modulation and intensitymodulation and generating a light emission instruction signal; an erroramplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein:the plurality of pulses output from said pulse widthmodulation means have a predetermined mutual relationship; and saidpulse width modulation and intensity modulation signal generating unit,said error amplifier and said current driving unit are formed as onechip of an integrated circuit.
 101. A semiconductor laser controlsystem, comprising:a pulse width modulation and intensity modulationsignal generating unit comprising pulse generating means for generatinga plurality of pulses having a frequency equal to a frequency of aninput clock signal and having different phases, the phase differencebeing a fixed phase difference, data converting means for convertinginput image data into pulse width modulation data and intensitymodulation data and pulse width modulation means for generating aplurality of pulses, which have undergone pulse width modulation basedon the pulse width modulation data, from the pulses generated by saidpulse generating means, said pulse width modulation and intensitymodulation signal generating unit performing pulse width modulation andintensity modulation and generating a light emission instruction signal;an error amplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein:at least one pulse of the plurality of pulses output fromsaid pulse width modulation means is always longer than others of saidplurality of pulses by said fixed phase difference; and said pulse widthmodulation and intensity modulation signal generating unit, said erroramplifier and said current driving unit are formed as one chip of anintegrated circuit.
 102. A semiconductor laser control system,comprising:a pulse width modulation and intensity modulation signalgenerating unit comprising pulse generating means for generating aplurality of pulses having a frequency equal to a frequency of an inputclock signal and having different phases, the phase difference being afixed phase difference, data converting means for converting input imagedata into pulse width modulation data and intensity modulation databased on the image data, a dot position control signal and a frequencyselecting signal, and pulse width modulation means for generating aplurality of pulses, which have undergone pulse width modulation basedon the pulse width modulation data, from the pulses generated by saidpulse generating means, said pulse width modulation and intensitymodulation signal generating unit performing pulse width modulation andintensity modulation and generating a light emission instruction signal;an error amplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, wherein:output mode clock frequencies comprise a first clockfrequency which is equal to the input clock frequency and second clockfrequency which is double the input clock frequency, one of said firstclock frequency and said second clock frequency being selected accordingto the frequency selecting signal, 1/2 the bits of each image data beingused for each dot writing and the number of tone levels being 1/2 thatof an equal frequency mode case when a double frequency mode isselected; and said pulse width modulation and intensity modulationsignal generating unit, said error amplifier and said current drivingunit are formed as one chip of an integrated circuit.
 103. Thesemiconductor laser control system according to claim 102, wherein, onlyin said double frequency mode, the image data comprises two sets ofM-bit data series, each of said two sets representing 2^(M) outputstates which include a one-dot full turn on state, a one-dot full turnoff state, and 2^(M-1) -1 middle levels, each of said middle levelshaving two levels of dot phase states.
 104. A semiconductor lasercontrol system, comprising:pulse width modulation and intensitymodulation signal generating unit comprising pulse generating means forgenerating a plurality of pulses having a frequency equal to a frequencyof an input clock signal and having different phases, the phasedifference being a fixed phase difference, data converting means forconverting input image data into pulse width modulation data andintensity modulation data based on the image data, a dot positioncontrol signal and a frequency selecting signal, and pulse widthmodulation means for generating a plurality of pulses, which haveundergone pulse width modulation based on the pulse width modulationdata, from the pulses generated by said pulse generating means, saidpulse width modulation and intensity modulation signal generating unitperforming pulse width modulation and intensity modulation andgenerating a light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, wherein:output modeclock frequencies comprise a first clock frequency which is equal to theinput clock frequency and a second clock frequency which is double theinput clock frequency, one of said first clock frequency and said secondclock frequency being selected according to the frequency selectingsignal, each of two bits of input image data being used as the dotposition control signal of writing dot; and said pulse width modulationand intensity modulation signal generating unit, said error amplifierand said current driving unit are formed as one chip of an integratedcircuit.
 105. A semiconductor laser control system, comprising:pulsewidth modulation and intensity modulation signal generating unitcomprising pulse generating means for generating a plurality of pulseshaving a frequency equal to a frequency of an input clock signal andhaving different phases, the phase difference being a fixed phasedifference, data converting means for converting input image data intopulse width modulation data and intensity modulation data and pulsewidth modulation means for generating a plurality of pulses, which haveundergone pulse width modulation based on the pulse width modulationdata, from the pulses generated by said pulse generating means, saidpulse width modulation and intensity modulation signal generating unitperforming pulse width modulation and intensity modulation andgenerating a light emission instruction signal; an error amplifierproviding a negative feedback loop together with a semiconductor laserand a light reception device which monitors light output of saidsemiconductor laser, said error amplifier controlling forward current ofsaid semiconductor laser so that a light reception signal proportionalto the light output of said semiconductor laser is equal to the lightemission instruction signal; and a current driving unit providing adriving current, according to the light emission instruction signal, toflow through said semiconductor laser as the forward current, thedriving current being generated so as to control driving of saidsemiconductor laser with a current of one of a difference and sum withthe control current of said negative feedback loop, wherein:the imagedata comprises an N-bit data series for representing 2^(N) output stateswhich include a one-dot full turn on state, a one-dot full turn offstate, and 2^(N-1) -1 middle levels, each of said middle levels havingtwo levels of dot phase states; and said pulse width modulation andintensity modulation signal generating unit, said error amplifier andsaid current driving unit are formed as one chip of an integratedcircuit.
 106. A semiconductor laser control system comprising:a pulsewidth modulation and intensity modulation signal generating unit which,based on input data, performs pulse width modulation and intensitymodulation and generates a light emission instruction signal; an erroramplifier providing a negative feedback loop together with asemiconductor laser and a light reception device which monitors lightoutput of said semiconductor laser, said error amplifier controllingforward current of said semiconductor laser so that a light receptionsignal proportional to the light output of said semiconductor laser isequal to the light emission instruction signal; and a current drivingunit providing a driving current, according to the light emissioninstruction signal, to flow through said semiconductor laser as theforward current, the driving current being generated so as to controldriving of said semiconductor laser with a current of one of adifference and sum with the control current of said negative feedbackloop, a differential quantum efficiency detecting unit for detecting thedifferential quantum efficiency of said semiconductor laser; a memoryunit for storing a detection result of said differential quantumefficiency detecting unit; an adding current setting unit for setting acurrent, corresponding to the light emission instruction signal, usingthe detection result stored in said memory unit; and a timing generatingunit, wherein:in initialization, said timing generating unit generates atiming signal slower than a control speed of said error amplifier, saiddifferential quantum efficiency detecting unit detects the differentialquantum efficiency of said semiconductor laser based on the timingsignal, said memory units storing a detection result at each timing, andthe current corresponding to the light emission instruction signal isset using the stored detection results; said current driving unitcomprises a high-speed voltage shifting unit in said error amplifier,includes a differential circuit for changing an amount of voltage shift,and is provided in said negative feedback loop; output mode change-overmeans is provided for selecting one of different clock-frequency outputmodes according to a frequency selecting signal; and said pulse widthmodulation and intensity modulation signal generating unit, said erroramplifier, said current driving unit, said differential quantumdetecting unit, said memory unit, said adding current setting unit, saidtiming generating unit and said output mode change-over means are formedas one chip of an integrated circuit by bipolar transistors.